gfx/drivers/metal: fold metal_common.h into metal.m

metal_common.h was a 420-line header included from exactly three
translation units: metal.m itself, ui/drivers/ui_cocoa.m, and
ui/drivers/ui_cocoatouch.m.  Of those, only metal.m used more than a
single declaration -- both UI files only needed the empty
@interface MetalView : MTKView @EnD forward declaration to type the
result of [MetalView new] and a (MetalView *) cast on _renderView.
The other ~415 lines (Context, FrameView, MetalDriver, MenuDisplay,
HDR machinery, RPixelFormat enum, the slang_process.h transitive
include, etc.) were dead weight on the two UI translation units.

Move the header's content into metal.m where it was already used as
a single TU via griffin_objc.m's umbrella, and replace the includes
in the two UI files with the three-line MetalView forward decl plus
<MetalKit/MetalKit.h>.  No public Metal driver C/Obj-C symbols are
referenced outside metal.m, so nothing else needs to change.

Optimisations enabled by the fold:

  - The four file-scope C functions exposed only for "external"
    consumers that did not exist (RPixelFormatToBPP, matrix_proj_ortho,
    glslang_format_to_metal, SelectOptimalPixelFormat) become static.
    glslang_format_to_metal and SelectOptimalPixelFormat get matching
    static forward decls near the top of the file because their
    definitions sit below their first call sites.

  - Three properties that were declared in the public header surface
    for downstream consumers that never materialised are dead in the
    closed system and have been removed:

      Context.drawableFormat       (getter returned _layer.pixelFormat)
      Context.hdrOffscreenFormat   (getter returned _hdrOffscreenFormat)
      FrameView.shaderEmitsHDR10
      FrameView.shaderEmitsHDR16

    The backing ivars _hdrOffscreenFormat, _shaderEmitsHDR10, and
    _shaderEmitsHDR16 are kept -- they are still read inside the
    owning class.  Only the public Obj-C accessors and their @Property
    declarations are gone.

  - RETRO_BEGIN_DECLS / RETRO_END_DECLS dropped along with the
    retro_common_api.h include they required (no public C API left).

  - Foundation/Metal/MetalKit/QuartzCore umbrella imports collapsed to
    the set already present at the top of metal.m.

The two pkg/apple/RetroArch_Metal.xcodeproj entries for metal_common.h
are removed.  The header had two PBXFileReference IDs in the project
(one declared, one dangling under the metal/ group); both group
children entries and the lone declaration are dropped.

Diff: +391 / -446 metal.m, -420 metal_common.h, -8/-9 ui_cocoa{,touch}.m,
      -3 project.pbxproj.  Net -61 lines.

griffin/griffin_objc.m only #imports metal.m and is unaffected.  The
Makefile.common metal.o rule and the per-file -fobjc-arc override in
the top-level Makefile (metal.m only) are unchanged; MRC-compiled
ui_cocoa{,touch}.m gain a tiny @interface MetalView : MTKView @EnD
that has no methods or properties and so compiles identically under
either memory model.

Author: LibretroAdmin