!UPSTREAM: meta: fix meta clear of layered framebuffers

TODO:
- Improve commit message.
WIP: refactor blorp clearing code in preparation for adding layered clears
i965: Fix blorp clear of layered framebuffers.

Fixes piglit test "spec/!OpenGL 3.2/layered-rendering/clear-color".
i965: Fix fast clear of depth buffers.

Fixes piglit test "spec/!OpenGL 3.2/layered-rendering/clear-depth".