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/compositor-wayland.c b/src/compositor-wayland.c
index ca747c4..7e59ecd 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -1962,12 +1962,7 @@
 	}
 
 	if (!c->use_pixman) {
-		if (gl_renderer->supports(&c->base, "wayland") < 0) {
-			weston_log("No support for "
-			           "EGL_{KHR,EXT,MESA}_platform_wayland; "
-			           "falling back to pixman.\n");
-			c->use_pixman = 1;
-		} else if (gl_renderer->create(&c->base,
+		if (gl_renderer->create(&c->base,
 					       EGL_PLATFORM_WAYLAND_KHR,
 					       c->parent.wl_display,
 					       gl_renderer->alpha_attribs,