gl-renderer: add shader bit input_is_premult

Add a new shader requirements bit input_is_premult which says whether
the texture sampling results in premultiplied alpha or not. Currently
this can be deduced fully from the shader texture variant, but in the
future there might a protocol extension to explicitly control it. Hence
the need for a new bit.

yuva2rgba() is changed to produce straight alpha always. This makes
sample_input_texture() sometimes produce straight or premultiplied
alpha. The input_is_premult bit needs to match sample_input_texture()
behavior. Doing this should save three multiplications in the shader for
straight alpha formats.

pipeline_premult() function is added, because in the future another
pipeline working on straight alpha will be needed.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>