Add a glfwSwapBuffersWithDamage() symbol

This new API helps the user’s compositor reduce memory bandwidth and
thus power usage by only re-rendering a bunch of dirty rectangles
instead of the entire application buffer.

There is no guarantee that it will effectively get used, it is perfectly
valid to continue damaging the entire buffer instead like with
glfwSwapBuffers() in some cases.

src/context.c

@@ -657,6 +657,26 @@ GLFWAPI void glfwSwapBuffers(GLFWwindow* handle)
     window->context.swapBuffers(window);
 }
 
+GLFWAPI void glfwSwapBuffersWithDamage(GLFWwindow* handle, GLFWrect* rects, int n_rects)
+{
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
+    _GLFW_REQUIRE_INIT();
+
+    if (window->context.client == GLFW_NO_API)
+    {
+        _glfwInputError(GLFW_NO_WINDOW_CONTEXT,
+                        "Cannot swap buffers of a window that has no OpenGL or OpenGL ES context");
+        return;
+    }
+
+    if (window->context.swapBuffersWithDamage)
+        window->context.swapBuffersWithDamage(window, rects, n_rects);
+    else
+        window->context.swapBuffers(window);
+}
+
 GLFWAPI void glfwSwapInterval(int interval)
 {
     _GLFWwindow* window;

src/internal.h

@@ -78,6 +78,7 @@ typedef struct _GLFWmutex       _GLFWmutex;
 
 typedef void (* _GLFWmakecontextcurrentfun)(_GLFWwindow*);
 typedef void (* _GLFWswapbuffersfun)(_GLFWwindow*);
+typedef void (* _GLFWswapbufferswithdamagefun)(_GLFWwindow*,GLFWrect*,int);
 typedef void (* _GLFWswapintervalfun)(int);
 typedef int (* _GLFWextensionsupportedfun)(const char*);
 typedef GLFWglproc (* _GLFWgetprocaddressfun)(const char*);
@@ -350,6 +351,7 @@ struct _GLFWcontext
 
     _GLFWmakecontextcurrentfun  makeCurrent;
     _GLFWswapbuffersfun         swapBuffers;
+    _GLFWswapbufferswithdamagefun swapBuffersWithDamage;
     _GLFWswapintervalfun        swapInterval;
     _GLFWextensionsupportedfun  extensionSupported;
     _GLFWgetprocaddressfun      getProcAddress;