gl renderer: Ensure texture sizes arent 0

We already ceil() the given float texture sizes here, so if they are
valid, the result should definitely be > 0. Textures with size 0 can't
be properly used, especially not as render targets, where they will
trigger an assertion failure later in a glCheckFramebuffer call.

gsk/gl/gskgldriver.c

@@ -429,6 +429,9 @@ create_texture (GskGLDriver *self,
   int width = ceilf (fwidth);
   int height = ceilf (fheight);
 
+  g_assert (width > 0);
+  g_assert (height > 0);
+
   if (width >= self->max_texture_size ||
       height >= self->max_texture_size)
     {