This patch fixes an oversight in part 5 of this patch series that
could result in an incorrect UV rect being used for an external
texture that uses a custom UV rect.

When the texture is an external texture, the UV rect is not known when
the external surface descriptor is created, because external textures
are not resolved until the lock() callback is invoked at the start of
the frame render. To handle this, query the texture resolver for the
UV rect if it's an external texture, otherwise use the default UV rect.