glsl/linker: Refactor in preparation for adding more shader stages.

glsl/linker: Refactor in preparation for adding more shader stages.

Rather than maintain separately named arrays and counts for vertex,
geometry, and fragment shaders, just maintain these as arrays indexed
by the gl_shader_type enum.

v2: When there is neither a vertex nor a geometry shader, set
prog->LastClipDistanceArraySize = 0, and clarify that the values is
not used.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Brian Paul <brianp@vmware.com>