The idea here is that we can do more complex combinations and use that
to support texture-scale nodes or use fancy texture formats (suc as
YUV).
I'm not sure this is actually necessary, but for now it gives more
flexibility.
Instead of having a descriptor set per operation, we just have one
descriptor set and bind all our images into it.
Then the shaders get to use an index into the large texture array
instead.
Getting this to work - because it's a Vulkan extension that needs to be
manually enabled, even though it's officially part of Vulkan 1.2 - is
insane.
Vulkan renders text using VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA and
VK_BLEND_FACTOR_SRC_ALPHA, but that implies per-channel alpha
blending, which currently produces the wrong results when blending
glyphs with the images beneath them.
Use the default pipeline constructors, which implies using the
ONE and ONE_MINUS_SRC_ALPHA.
Pass the glyph position into the glyph caching functions,
not just the glyph index. This allows us to cache different
images for different subpixel positions.
