[cogl-material] Always bind the new texture handle

Cogl previously tried to cache the currently bound texture when
drawing through the material API to avoid excessive GL calls. However,
a few other places in Cogl and Clutter rebind the texture as well so
this can cause problems.

This was causing shaped windows to fail in Mutter because
ClutterGLXTexturePixmap was binding a different texture to update it
while the second texture unit was still active which meant the mask
texture would not be selected when the shaped window was drawn
subsequent times.

Ideally we would fix this by providing a wrapper around glBindTexture
which would affect the cached value. The cache would also have to be
cleared if a selected texture was deleted.

clutter/cogl/common/cogl-material.c

@@ -949,18 +949,20 @@ _cogl_material_flush_layers_gl_state (CoglMaterial *material,
 
       GE (glActiveTexture (GL_TEXTURE0 + i));
 
-      if (!gl_layer_info
+      /* FIXME: We could be more clever here and only bind the texture
-          || gl_layer_info->gl_target != gl_target
+         if it is different from gl_layer_info->gl_texture to avoid
-          || gl_layer_info->gl_texture != gl_texture)
+         redundant GL calls. However a few other places in Cogl and
-        {
+         Clutter call glBindTexture such as ClutterGLXTexturePixmap so
+         we'd need to ensure they affect the cache. Also deleting a
+         texture should clear it from the cache in case a new texture
+         is generated with the same number */
 #ifdef HAVE_COGL_GLES2
-          cogl_gles2_wrapper_bind_texture (gl_target,
+      cogl_gles2_wrapper_bind_texture (gl_target,
-                                           gl_texture,
+                                       gl_texture,
-                                           gl_internal_format);
+                                       gl_internal_format);
 #else
-          GE (glBindTexture (gl_target, gl_texture));
+      GE (glBindTexture (gl_target, gl_texture));
 #endif
-        }
 
       /* Disable the previous target if it was different */
       if (gl_layer_info &&