i965: Fix clears of layered framebuffers with mismatched layer counts.

Previously, Mesa enforced the following rule (from
ARB_geometry_shader4's list of criteria for framebuffer completeness):

  * If any framebuffer attachment is layered, all attachments must have
    the same layer count.  For three-dimensional textures, the layer count
    is the depth of the attached volume.  For cube map textures, the layer
    count is always six.  For one- and two-dimensional array textures, the
    layer count is simply the number of layers in the array texture.
    { FRAMEBUFFER_INCOMPLETE_LAYER_COUNT_ARB }

However, when ARB_geometry_shader4 was adopted into GL 3.2, this rule
was dropped; GL 3.2 permits different attachments to have different
layer counts.  This patch brings Mesa in line with GL 3.2.

In order to ensure that layered clears properly clear all layers, we
now have to keep track of the maximum number of layers in a layered
framebuffer.

Fixes the following piglit tests in spec/!OpenGL 3.2/layered-rendering:
- clear-color-all-types 1d_array mipmapped
- clear-color-all-types 1d_array single_level
- clear-color-mismatched-layer-count
- framebuffer-layer-count-mismatch