Allow to specify a D₂ transform when attaching a texture to a
subsurface, to handle flipped and rotated content. The Wayland
implementation handles these transforms by setting a buffer
transform on the subsurface.
All callers have been updated to pass GDK_TEXTURE_TRANSFORM_NORMAL.
This will let us use a subset of the full texture, which can
be necessary in the case that converters put padding around
content in dmabufs. The naming follows the Wayland viewporter
spec.
For now, make all callers pass the full texture rect.
