meta: fix meta clear of layered framebuffers

From section 4.4.7 (Layered Framebuffers) of the GLSL 3.2 spec:

    When the Clear or ClearBuffer* commands are used to clear a
    layered framebuffer attachment, all layers of the attachment are
    cleared.

This patch fixes meta clears to properly clear all layers of a layered
framebuffer attachment.  We accomplish this by adding a geometry
shader to the meta clear program which sets gl_Layer to a uniform
value.  When clearing a layered framebuffer, we execute in a loop,
setting the uniform to point to each layer in turn.
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".
!UPSTREAM: main: check texture target when validating layered framebuffers.

From section 4.4.4 (Framebuffer Completeness) of the GL 3.2 spec:

    If any framebuffer attachment is layered, all populated
    attachments must be layered. Additionally, all populated color
    attachments must be from textures of the same target.

We weren't checking that the attachments were from textures of the
same target.

TODO:
- Make a piglit test for this, and validate it on Nvidia.