This generalizes the overriding mechanism in gen_Vk_format_table.py.
Instead of defining overrides inline, we keep the vk_format_map as a
1:1 mapping from angle format to vk format, and use the overrides map
to specify any additional emulation or features we need to support
OpenGL textures on Vulkan.

This will simplify the implementation for Depth/Stencil formats by
making the generator script a bit easier to work with.

No code changes, generator script refactor only.

Bug: angleproject:2357
Change-Id: I7132d991795eb91d6ad838481c52545c891215bc
Reviewed-on: https://chromium-review.googlesource.com/957433Reviewed-by: Frank Henigman <fjhenigman@chromium.org>