Add a pl_gpu implementation that uses Direct3D 11. This uses SPIRV-Cross
to translate shaders from GLSL to HLSL. In its current state, plplay
works and the test suite passes with feature level 10_0 and up. (9_x is
unlikely to ever pass due to runtime-enforced shader complexity limits.)

Missing features include emulated texture formats, cached_program,
`pl_gpu_limits.thread_safe`, and HDR/high bit depth support in
pl_swapchain, though these shouldn't be too hard to implement.

This also updates the GLFW demos to be able to use Direct3D 11 through
GLFW_NO_API.

Closes #18