Implement libretro core (#1215)

* libretro core

* Bringing citra libretro implementation over
* libretro: hook up vulkan renderer
* libretro: github actions
* libretro: gyro
* libretro: core options v2
* libretro: on ios turn off shader jit if unavailable
* moltenvk 1.3.0 introduces 8-bit indexes but allocates 16-bit for metal; this ends up allocating stream buffer * 2 = 132MiB. Instead, just use 16-bit indexes. (This will be necessary for standalone when bumping moltenvk version.)

* libretro core: address review feedback

* libretro: microphone support

* cmake: Add ENABLE_ROOM_STANDALONE to list of incompatible libretro flags

* libretro: proper initial geometry

* libretro: fix software renderer

* libretro: address review feedback

* .github/libretro.yml: Pin macOS runners at macOS 26

* ci: Remove explicit selection of Xcode 16.0

* .github/libretro.yml: remove unnecessary windows builder apt commands

* .github/libretro.yml: bump min macos version to 11.0

* ci: Re-enable CI jobs for all libretro cores

This is under the condition that we don't introduce build cache for these builds

---------

Co-authored-by: OpenSauce04 <opensauce04@gmail.com>
Co-authored-by: PabloMK7 <hackyglitch2@gmail.com>

.gitmodules

@@ -103,3 +103,6 @@
 [submodule "externals/xxHash"]
 	path = externals/xxHash
 	url = https://github.com/Cyan4973/xxHash.git
+[submodule "externals/libretro-common"]
+	path = externals/libretro-common/libretro-common
+	url = https://github.com/libretro/libretro-common.git