shaders/sampling: allow sampling arbitrary components

This replaces `int components` by a new `uint8_t component_mask`, which
is more flexible. I decided not to deprecate the original field, because
it doesn't really cause any harm to continue supporting, and in most
cases you don't need the flexibility provided by a mask, so it's just a
hassle.

Made for the sake of allowing us to more efficiently render sub-sampled
planes without needing to waste GPU time scaling unnecessary components
like luma.