EGL: Use EGL_EXT_present_opaque when available

This extensions allows GLFW to instruct the driver to ignore the alpha
bits, even in formats which contain them.  This makes it possible to use
the alpha bits as extra storage, without it affecting the end result
getting displayed to the user.

Fixes #1434
Fixes #1803

(cherry picked from commit 6281f498c8)

src/egl_context.c

@@ -426,6 +426,8 @@ GLFWbool _glfwInitEGL(void)
         extensionSupportedEGL("EGL_KHR_get_all_proc_addresses");
     _glfw.egl.KHR_context_flush_control =
         extensionSupportedEGL("EGL_KHR_context_flush_control");
+    _glfw.egl.EXT_present_opaque =
+        extensionSupportedEGL("EGL_EXT_present_opaque");
 
     return GLFW_TRUE;
 }
@@ -599,6 +601,9 @@ GLFWbool _glfwCreateContextEGL(_GLFWwindow* window,
     if (!fbconfig->doublebuffer)
         setAttrib(EGL_RENDER_BUFFER, EGL_SINGLE_BUFFER);
 
+    if (_glfw.egl.EXT_present_opaque)
+        setAttrib(EGL_PRESENT_OPAQUE_EXT, !fbconfig->transparent);
+
     setAttrib(EGL_NONE, EGL_NONE);
 
     window->context.egl.surface =