Crash using Metal renderer when using RGB5A1 textures

[zacbrannelly]

Describe the bug

In the Metal renderer, a simple call to createTexture2D to create a RGB5A1 texture has side effects that breaks reset, specifically when the depth buffer is resized.

After calling createTexture2D with a pixel format that uses texture swizzle which applies swizzle that isn't just R -> R, G -> G, B -> B, A -> A (for RGB5A1 it is R -> B, G -> G, B -> R, A -> A), then I get the following error when calling reset, which I call after window resize:

-[MTLTextureDescriptorInternal validateWithDevice:]:1357: failed assertion `Texture Descriptor Validation
Texture swizzling is not compatable with MTLTextureUsageRenderTarget

To Reproduce

bgfx::createTexture2D(100, 100, false, 1, RGB5A1, BGFX_TEXTURE_NONE, NULL);
bgfx::reset(1920, 1080, 0);

Expected behavior

reset to complete without hitting an assertion.

Additional context

I can fix the problem by editing the SwapChainMtl::resize function in renderer_mtl.mm, where I change this:

if (s_renderMtl->m_hasPixelFormatDepth32Float_Stencil8)
{
    desc.pixelFormat = MTLPixelFormatDepth32Float_Stencil8;
}

To this:

if (s_renderMtl->m_hasPixelFormatDepth32Float_Stencil8)
{
    desc.pixelFormat = MTLPixelFormatDepth32Float_Stencil8;
    desc.swizzle = { MTLTextureSwizzleRed, MTLTextureSwizzleGreen, MTLTextureSwizzleBlue, MTLTextureSwizzleAlpha };
}

Happy to make a PR but cognisant of the fact I'm not entirely sure what/if there are any other issues this would cause.

[mcourteaux]

No, the PR would be good. Just apply the swizzle always, instead of inside the if body.
The TextureDescription is being reused to avoid allocations, which is why this causes the swizzle to be wrong. Basically we should treat all fields as uninitialised garbage. So while you're at it, go through the list of members in TextureDescription and make sure they are all set properly, in all cases.