Improve MRT support

[mvaligursky]

Issue for initial implementation of MRT: #4230

Possible improvements:

• Per-attachment clear colors and clear flags on the render pass. RenderPass#setClearColor accepts an optional attachment index, and WebGL clears the color attachments individually using clearBufferfv, applying their own clear colors and clear flags. Previously a single gl.clear applied attachment 0's clear color to all attachments, and ignored the per-attachment clear flags. Done in Add per-attachment clear colors for multiple render targets #9142.
• Per-attachment blend state and color write mask. BlendState#setAttachment gives an individual color attachment its own blend state and write mask - supported natively on WebGPU, and using the OES_draw_buffers_indexed extension on WebGL2, with GraphicsDevice#supportsIndependentBlending reporting the support. Done in Add independent blending for multiple render targets #9140.
• Expand the API on the camera to specify the clear color per color attachment, and use those with MRT.
• Expand the layer clear color flag to be per color attachment. Note that the mid-render-pass clear this uses (GraphicsDevice#clear) is implemented differently per platform, and the platforms currently disagree on MRT: WebGL uses gl.clear which applies a single color to all attachments (and does not support integer formats), while WebGPU uses a fullscreen quad (WebgpuClearRenderer) with a float color output, which only writes attachment 0. Converging these means per-attachment clearBuffer* calls on WebGL, and an MRT-aware clear shader on WebGPU. Also blocked on the camera API above, as the layer flags are booleans and the clear color currently comes from the camera, which only has a single color.
• Clearing of integer format color attachments at the render pass start (clearBufferiv / clearBufferuiv on WebGL - WebGPU supports this natively via clearValue), which would also allow the Picker to use a uint32 format instead of encoding the id into RGBA8. Note the picker migration additionally needs uint fragment outputs for the pick shader variants. Mid-render-pass integer clears (layers) are excluded - on WebGPU the clear shader would need integer output variants, and nothing currently needs it. Done in Add clear support for integer format color attachments #9143.