meta: fix meta clear of layered framebuffers

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.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>