op: texture_view,read/write

[kainino0x]

Some planning is needed here, then maybe split up the task into smaller tasks.

• Make sure reads/writes go to the correct subresources of texture views.
• Stencil reads return S,X,X,X (warn if not S,S,S,S or S,0,0,1)
• Gather operations
• blocked on texture_2d_depth for loading depth values might be unnecessary? gpuweb#2094
  • Depth reads using texture_2d return D,X,X,X (warn if not D,D,D,D or D,0,0,1)
  • Gather operations on depth/stencil components 2,3,4

Can add basic tests, but full tests will benefit from the generalized texture write helpers which is more complex. #881

[austinEng]

Some drivers have difficulty with the following. Particularly with depth-stencil textures. We should make sure it has some test coverage:

• Making a texture view and then using it (sampling, depth-stencil attachment), offset at a non-zero mip level or array layer
• Making a texture view, and sampling from it using textureLoad on texture_2d_array in the shader with various zero, and nonzero array_index and level args.

[austinEng]

this is in the TODOs in the tests already, but from code cov, we also need to test all dimensions (untested: 1d/3d/cube/cube-array)