Add glfwGetWindowOpacity and glfwSetWindowOpacity

This adds support for setting the opacity of the whole window, including
any decorations.

Fixes #1089.

tests/CMakeLists.txt

@@ -32,6 +32,7 @@ add_executable(gamma WIN32 MACOSX_BUNDLE gamma.c ${GLAD})
 add_executable(icon WIN32 MACOSX_BUNDLE icon.c ${GLAD})
 add_executable(inputlag WIN32 MACOSX_BUNDLE inputlag.c ${GETOPT} ${GLAD})
 add_executable(joysticks WIN32 MACOSX_BUNDLE joysticks.c ${GLAD})
+add_executable(opacity WIN32 MACOSX_BUNDLE opacity.c ${GLAD})
 add_executable(tearing WIN32 MACOSX_BUNDLE tearing.c ${GETOPT} ${GLAD})
 add_executable(threads WIN32 MACOSX_BUNDLE threads.c ${TINYCTHREAD} ${GLAD})
 add_executable(timeout WIN32 MACOSX_BUNDLE timeout.c ${GLAD})
@@ -45,8 +46,8 @@ if (RT_LIBRARY)
     target_link_libraries(threads "${RT_LIBRARY}")
 endif()
 
-set(WINDOWS_BINARIES empty gamma icon inputlag joysticks tearing threads
-                     timeout title windows)
+set(WINDOWS_BINARIES empty gamma icon inputlag joysticks opacity tearing
+                     threads timeout title windows)
 set(CONSOLE_BINARIES clipboard events msaa glfwinfo iconify monitors reopen
                      cursor)
 
@@ -75,6 +76,7 @@ if (APPLE)
     set_target_properties(gamma PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Gamma")
     set_target_properties(inputlag PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Input Lag")
     set_target_properties(joysticks PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Joysticks")
+    set_target_properties(opacity PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Opacity")
     set_target_properties(tearing PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Tearing")
     set_target_properties(threads PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Threads")
     set_target_properties(timeout PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Timeout")

tests/opacity.c

@@ -0,0 +1,106 @@
+//========================================================================
+// Window opacity test program
+// Copyright (c) Camilla Löwy <elmindreda@glfw.org>
+//
+// This software is provided 'as-is', without any express or implied
+// warranty. In no event will the authors be held liable for any damages
+// arising from the use of this software.
+//
+// Permission is granted to anyone to use this software for any purpose,
+// including commercial applications, and to alter it and redistribute it
+// freely, subject to the following restrictions:
+//
+// 1. The origin of this software must not be misrepresented; you must not
+//    claim that you wrote the original software. If you use this software
+//    in a product, an acknowledgment in the product documentation would
+//    be appreciated but is not required.
+//
+// 2. Altered source versions must be plainly marked as such, and must not
+//    be misrepresented as being the original software.
+//
+// 3. This notice may not be removed or altered from any source
+//    distribution.
+//
+//========================================================================
+
+#include <glad/glad.h>
+#include <GLFW/glfw3.h>
+
+#define NK_IMPLEMENTATION
+#define NK_INCLUDE_FIXED_TYPES
+#define NK_INCLUDE_FONT_BAKING
+#define NK_INCLUDE_DEFAULT_FONT
+#define NK_INCLUDE_DEFAULT_ALLOCATOR
+#define NK_INCLUDE_VERTEX_BUFFER_OUTPUT
+#define NK_INCLUDE_STANDARD_VARARGS
+#include <nuklear.h>
+
+#define NK_GLFW_GL2_IMPLEMENTATION
+#include <nuklear_glfw_gl2.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+
+static void error_callback(int error, const char* description)
+{
+    fprintf(stderr, "Error: %s\n", description);
+}
+
+int main(int argc, char** argv)
+{
+    GLFWmonitor* monitor = NULL;
+    GLFWwindow* window;
+    struct nk_context* nk;
+    struct nk_font_atlas* atlas;
+
+    glfwSetErrorCallback(error_callback);
+
+    if (!glfwInit())
+        exit(EXIT_FAILURE);
+
+    window = glfwCreateWindow(400, 400, "Opacity", NULL, NULL);
+    if (!window)
+    {
+        glfwTerminate();
+        exit(EXIT_FAILURE);
+    }
+
+    glfwMakeContextCurrent(window);
+    gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
+    glfwSwapInterval(1);
+
+    nk = nk_glfw3_init(window, NK_GLFW3_INSTALL_CALLBACKS);
+    nk_glfw3_font_stash_begin(&atlas);
+    nk_glfw3_font_stash_end();
+
+    while (!glfwWindowShouldClose(window))
+    {
+        int width, height;
+        struct nk_rect area;
+
+        glfwGetWindowSize(window, &width, &height);
+        area = nk_rect(0.f, 0.f, (float) width, (float) height);
+
+        glClear(GL_COLOR_BUFFER_BIT);
+        nk_glfw3_new_frame();
+        if (nk_begin(nk, "", area, 0))
+        {
+            float opacity = glfwGetWindowOpacity(window);
+            nk_layout_row_dynamic(nk, 30, 2);
+            if (nk_slider_float(nk, 0.f, &opacity, 1.f, 0.001f))
+                glfwSetWindowOpacity(window, opacity);
+            nk_labelf(nk, NK_TEXT_LEFT, "%0.3f", opacity);
+        }
+
+        nk_end(nk);
+        nk_glfw3_render(NK_ANTI_ALIASING_ON);
+
+        glfwSwapBuffers(window);
+        glfwWaitEventsTimeout(1.0);
+    }
+
+    nk_glfw3_shutdown();
+    glfwTerminate();
+    exit(EXIT_SUCCESS);
+}
+