mesa: Implement the GL entry points defined by ARB_shader_image_load_store.

mesa: Implement the GL entry points defined by ARB_shader_image_load_store.

v2: Name image format classes consistently, fix array and 3D teximage
    selection with layered = GL_FALSE, make sure that the
    user-specified layer is less than the number of texture layers,
    add some asserts.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>