In future commits, we want to be able to handle more complex textures,
such as video frames which are encoded in a YUV-pixel format and have
multiple planes (which each map to a separate texture).
To accomplish this, we introduce a new object `MetaMultiTexture`: this
object can deal with more complex formats by handling multiple
`CoglTexture`s.
It supports shaders for pixel format conversion from YUV to RGBA, as
well as blending. While custom bleding is currently only required for
YUV formats, we also implement it for RGB ones. This allows us to
simplify code in other places and will be needed in the future once
we want to support blending between different color spaces.
Co-Authored-By: Robert Mader <robert.mader@collabora.com>
Co-Authored-By: Sebastian Wick <sebastian.wick@redhat.com>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2191>
