gallium: Add pipe_image_view::single_layer_view

OpenGL has a goofy feature that allows creating an image view of a single layer of an array texture... in which case that image is treated as non-arrayed in shader. If you have a 16x16x16 3D texture and bind the third layer, you get a 16x16 2D texture instead of a 16x16x1 3D texture. That distinction matters to the hardware on AGX, since the texture dimension needs to match between the shader and the pipe_image_view. If the shader is going to use image2D, we need to know that the pipe_image_view should be treated as 2D (even though the underlying resource is 3D).

"But, Alyssa, we already have first_layer and last_layer. Surely you can just check if first_layer == last_layer?" you ask. The problem is that doesn't distinguish a 16x16x1 3D texture (accessed as image3D in the shader) from a 16x16 slice (accessed as image2D in the shader) of a 16x16x16 3D texture. To solve, we add a boolean flag indicating we want to create a view (with a lower dimension than the underlying resource). This provides an unambiguous way to communicate this case to drivers.

Signed-off-by: Alyssa Rosenzweig alyssa@rosenzweig.io