Support MSAA: multisampled textures and resolve targets

[botoddly]

Closes #626.

Changes

• GpuDevice.CreateTexture, CreateColorTargetTexture and CreateDepthBufferTexture take an optional SampleCount. GpuDevice.IsSampleCountSupported checks a count for a format. A multisampled texture can only be a color or depth-stencil target, so CreateColorTargetTexture drops the sampler usage for it, and CreateDepthBufferTexture throws with sampler: true.
• Texture has SampleCount and Usage. Memory stats count every sample. SubmitAndDownloadTexture refuses a multisampled texture.
• RenderPassBuilder.AddColorTarget(texture, resolveTexture[, settings]) names the texture a color target resolves into. CommandBuffer.CreateRenderPass takes an optional span of resolve textures.
• Building a pass throws when the attachments' sample counts differ, when a store operation and its resolve texture disagree, when a resolve texture breaks SDL's rules (one sample, the target's format and size, ColorTarget usage), and when a depth-stencil buffer's store operation resolves.
• GraphicsPipelineBuilder.Build() throws for a sample count the device does not support with a target format. RenderPass.BindGraphicsPipeline throws when the pipeline's sample count differs from the pass's.
• SampleCount moved to its own file.
• New docs/multisampling.md, linked from AGENTS.md, render-pass-flow.md and pipeline-configuration.md. It covers what each backend supports and the matching depth-stencil buffer.
• New Pixely.Tutorials.Multisampling draws a triangle with 4 samples and resolves it into the swapchain texture.

Known limits

• The WebGPU fork reports 4 samples as supported for every format: SDL_GPUTextureSupportsSampleCount ignores the format on WebGPU XDL_wgpu#24.
• On Direct3D 12, the pinned SDL can report no multisampling for depth formats such as Depth24Stencil8: D3D12: MSAA gets disabled because it checks the wrong depth format libsdl-org/SDL#16124, fixed in a later SDL. Commented in GpuDevice.IsSampleCountSupported.
• The texture creation methods and CreateRenderPass gained optional parameters instead of new overloads, so code compiled against the old signatures needs a rebuild.
• EnableMultiSampling's mask must stay null: SDL reserves the sample mask.

Testing

• Pixely.Tests (469) and Pixely.Ui.Tests (490) pass.
• Headless with GPU validation on Vulkan (radv): the Multisampling tutorial's edges show the 1/4, 2/4 and 3/4 coverage shades, with no validation errors. Triangle, StencilBuffer, DepthOnly, ImageLoading and TextureArray still run cleanly.
• Not run on Direct3D 12, Metal or WebGPU.

[github-actions]

📦 Development package Pixely 0.0.116 is available.

Workflow run (attempt 1)

[github-actions]

✅ Development package validation passed on all platforms.

Workflow run (attempt 1)