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

src/egl_context.c

@@ -472,6 +472,8 @@ 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;
 }
@@ -646,6 +648,9 @@ 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);
 
     native = _glfw.platform.getEGLNativeWindow(window);

src/internal.h

@@ -177,6 +177,7 @@ typedef const GLubyte* (APIENTRY * PFNGLGETSTRINGIPROC)(GLenum,GLuint);
 #define EGL_CONTEXT_RELEASE_BEHAVIOR_FLUSH_KHR 0x2098
 #define EGL_PLATFORM_X11_EXT 0x31d5
 #define EGL_PLATFORM_WAYLAND_EXT 0x31d8
+#define EGL_PRESENT_OPAQUE_EXT 0x31df
 #define EGL_PLATFORM_ANGLE_ANGLE 0x3202
 #define EGL_PLATFORM_ANGLE_TYPE_ANGLE 0x3203
 #define EGL_PLATFORM_ANGLE_TYPE_OPENGL_ANGLE 0x320d
@@ -803,6 +804,7 @@ struct _GLFWlibrary
         GLFWbool        EXT_platform_base;
         GLFWbool        EXT_platform_x11;
         GLFWbool        EXT_platform_wayland;
+        GLFWbool        EXT_present_opaque;
         GLFWbool        ANGLE_platform_angle;
         GLFWbool        ANGLE_platform_angle_opengl;
         GLFWbool        ANGLE_platform_angle_d3d;