Enable minimal support of VRS in D3D11 and GL

[TheMostDiligent]

To make D3D11 and GL consistent with other backends, and to make Tutorial 24 run in D3D11 and GL modes with minimal configuration:

• Enable SHADING_RATE_CAP_FLAG_PER_DRAW capability with single SHADING_RATE_1X1 supported rate and SHADING_RATE_COMBINER_PASSTHROUGH combiner
• Use some default values for MinTileSize/MaxTileSize (e.g., 16/16)
• Create dummy shading rate texture
• SetShadingRate will be a noop.