Dawn Texel Copy Buffer Row Alignment

The dawn-texel-copy-buffer-row-alignment feature exposes the alignment restriction of byetsPerRow in wgpu::TexelCopyBufferLayout. Each backend may have its own min alignment value. Without this feature, the alignment must be 256 for all backends.

Additional functionalities:

• Adds wgpu::DawnTexelCopyBufferRowAlignmentLimits as chained struct for wgpu::SupportedLimits. It has a member minTexelCopyBufferRowAlignment to indicate the alignment limit of the current device.

Notes:

• Even with this feature enabled, the alignment clients actually use, still needs to respect ‘bytes-per-texel-block’, and should be the max of them.
• The feature currently is only available on D3D11 backend.