mesa/st: assert that lowering is supported

Some of these lowerings aren't supported for drivers that supports tesselation and geometry shaders. Let's add a couple of asserts to make it obvious if these have been enabled when it's not possible.

To make this a bit easier to read, refactor the code a little bit. This way we fist check if we're in the last vertex-outputting stage, which we already need to check for depth-clamp lowering anyway.

Signed-off-by: Erik Faye-Lund erik.faye-lund@collabora.com