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.
This introduces the rect object and adds a rect_distance() and
rect_coverage() function.
_distance() returns the signed distance tp the rectangle.
_coverage() returns the coverage of a pixel centered at that position.
Note that the pixel size is computed using dFdx/dFdy.
Move the resources of each renderer to its subdirectory.
We've previously done that for the ngl renderer, but it
is better to be consistent and do it for all the renderers.
