Introduce a new Texture type with DX,VK,MTL implementations.

[EmilioLaiso]

Summary

• Introduced Texture.h with a shared TextureCreateDesc, TextureFormat enum, TextureUsage flags, ClearValue variant (ClearColor / ClearDepthStencil), and validateTextureCreateDesc returning specific llvm::Error messages
• Added createTexture to all three backends (DX, Vulkan, Metal) as a virtual method on Device, handling resource creation, optimized clear values, and format/usage validation
• Unified render target creation via createRenderTargetFromCPUBuffer(Device&, CPUBuffer&) free function — all backends now route through createTexture instead of manually calling API-specific resource creation
• Unified depth/stencil creation via createDefaultDepthStencilTarget(Device&, Width, Height) free function with D32FloatS8Uint format — added depth testing to DX and Metal backends (previously only Vulkan had it)
• Readback buffers created through the virtual createBuffer API with MemoryLocation::GpuToCpu; Metal refactored from StorageModeShared texture with getBytes() to StorageModePrivate texture + blit copy to readback buffer (this is to align functionality between backends in preparation of api-agnostic code-flow. Sacrifices had to be made 😛)
• Views on textures — RTV/DSV descriptor heaps (DX) and VkImageView (VK) are now created at createTexture time for RenderTarget/DepthStencil usage, removing view creation boilerplate from pipeline setup
• Render target and readback buffer are stored directly as shared_ptr<VulkanTexture> / shared_ptr<VulkanBuffer> on InvocationState
• Clear values read from texture descriptors in all backends instead of being hardcoded
• Added toTextureFormat(DataFormat, Channels) as a bridge function while refactoring, with validateTextureDescMatchesCPUBuffer to catch mismatches between the old and new description systems
• Split Metal storage mode helpers into getMetalTextureStorageMode and getMetalBufferResourceOptions because GpuToCpu maps differently for textures (Managed) vs buffers (Shared)

Future direction

Added for context to better understand some of the changes in this PR.

• SRV/UAV views on textures — currently not stored on the texture because they would require a descriptor heap allocation scheme
• Work towards eliminating DataFormat + Channels dual description.
• Route VK vertex buffer through createBuffer — still uses the old ResourceRef/BufferRef system
• Abstract readback copy
• Unify InvocationState across backends

[manon-traverse]

NOTE: unmapping is not necessary in Vulkan or DX12. It's fine to leave it as is, but I thought it was good to know it's fine to leave this persistently mapped.

[manon-traverse]

Just a couple of small comments, but other than that looks good to me

[MarijnS95]

Fwiw in our codebase we use a custom heap that might not place this restriction on buffers.

[manon-traverse]

Note: we should try to store the platform-agnostic pointer here and downcast where needed, as the goal is to use these only in the platform-abstract way.

I am fine with keeping it this way for this PR, but it's something we should do in the future. Perhaps once we have an agnostic way of binding resources?