mame: upstream issue tracking for 0.252 and later: 3rd-party component bgfx broken for 10.14/10.15

[mascguy (Christopher Nielsen)]

Presently Mame is pinned at 0.251 for 10.14 and later, due to breakage within 3rd-party component BGFX. Specifically, certain pixel formats are unavailable in releases prior to Big Sur, even though upstream still officially supports 10.14 and later.

For 10.14:

renderer_mtl.mm:: error: unknown type name 'MTLTextureSwizzleChannels'
renderer_mtl.mm:: error: use of undeclared identifier 'MTLTextureSwizzleAlpha'
renderer_mtl.mm:: error: use of undeclared identifier 'MTLTextureSwizzleBlue'
renderer_mtl.mm:: error: use of undeclared identifier 'MTLTextureSwizzleGreen'
renderer_mtl.mm:: error: use of undeclared identifier 'MTLTextureSwizzleRed'

For 10.15:

renderer_mtl.mm: error: 'MTLPixelFormatABGR4Unorm' is unavailable: not available on macOS
renderer_mtl.mm: error: 'MTLPixelFormatB5G6R5Unorm' is unavailable: not available on macOS
renderer_mtl.mm: error: 'MTLPixelFormatBGR5A1Unorm' is unavailable: not available on macOS

This issue has officially been reported to upstream, and is being tracked by:

• ​11254 - bgfx: metal renderer: pixel-format build errors for macOS 10.14 and 10.15

While the issue would be easy enough to patch - it's limited to two source files - there's a possibility that it will cause downstream compilation (or runtime) issues with Mame. So rather than simply blindly patch it, the goal is to engage with upstream, and validate that we won't have other unexpected problems.