Add override for getCustomAttribute in MetalTextureGpu

[srmainwaring]

This PR implements an override for the function getCustomAttribute in the class MetalTextureGpu.

This function is may be used to access some of the render systems underlying objects. For Metal we are interested in getting the object id for a texture. The two named attributes that may be accessed are msFinalTextureBuffer and msMsaaTextureBuffer, for example:

void *mtlTexturePtr = nullptr;
texture->getCustomAttribute( "msFinalTextureBuffer", &mtlTexturePtr );
id<MTLTexture> mtlTexture = CFBridgingRelease( mtlTexturePtr );

[darksylinc]

Hi!

That is not the best way to transfer ptrs around in ARC.

Please do what MetalWindow::getCustomAttribute does which uses CFBridgingRetain:

*static_cast<void**>(pData) = (void*)CFBridgingRetain( mMetalView );

and is obtained via:

void *uiViewPtr = 0;
renderWindow->getCustomAttribute( "UIView", &uiViewPtr );
UIView *uiView = CFBridgingRelease( uiViewPtr );

Which is also used by MetalRenderSystem::_hlmsComputePipelineStateObjectCreated and _hlmsSamplerblockCreated

[srmainwaring]

@darksylinc thanks for the feedback - I've modified the PR and force pushed the change.