gl-renderer: call supports() automatically in create()

This means compositors don't need to call supports() manually and
create() will just return -1 in the failure case as before. This also
means we can deal with the case of eglGetProcAddress returning
non-NULL but not actually being available at runtime.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/src/gl-renderer.h b/src/gl-renderer.h
index 2434fdd..4208bf1 100644
--- a/src/gl-renderer.h
+++ b/src/gl-renderer.h
@@ -53,6 +53,8 @@
 #define EGL_PLATFORM_X11_KHR 0x31D5
 #endif
 
+#define NO_EGL_PLATFORM 0
+
 enum gl_renderer_border_side {
 	GL_RENDERER_BORDER_TOP = 0,
 	GL_RENDERER_BORDER_LEFT = 1,
@@ -64,9 +66,6 @@
 	const EGLint *opaque_attribs;
 	const EGLint *alpha_attribs;
 
-	int (*supports)(struct weston_compositor *ec,
-			const char *extension_suffix);
-
 	int (*create)(struct weston_compositor *ec,
 		      EGLenum platform,
 		      void *native_window,