We can't use normal border colors if we are doing linear filtering as we will sample slightly outside on the edges and blend to the border color.
Replace this with a way to push an arbitrary layer alpha and return that in the OOB case, and when sampling, clamp to edge.
This also may be more efficient to avoid sampling OOB a lot of the time in some cases.
Signed-off-by: Joshua Ashton <joshua@froggi.es>
Cleans up the composite shader significantly as well as it uses
arrays of descriptors/samplers now.
This all gets unrolled as the loop is spec-constant sized.
