glsl: Fix gl_type of usamplerCube built-in type.

I'm not aware of any piglit tests that this fixes, but the old code
was obviously wrong.
glsl: Don't return bad values from _mesa_shader_type_to_index.

This will avoid compiler warnings in the patch that follows.  There
should be no user-visible effect because the change only affects the
behaviour when an invalid enum is passed to
_mesa_shader_type_to_index(), and that can only happen if there is a
bug elsewhere in Mesa.
main: Move MESA_SHADER_TYPES outside of gl_shader_type enum.

This will avoid spurious compiler warnings in the patch that follows.
glsl: Replace _mesa_glsl_parser_targets enum with gl_shader_type.

These enums were redundant.
!UPSTREAM: i965/gs: Generalize some error messages for GS.

TODO: find more places to generalize.
TODO: Don't use "VS/GS" terminology.  Use "vec4" terminology, since that will endure longer.
!UPSTREAM: glsl/linker: Fail if linking geometry shader with 0 GEOMETRY_VERTICES_OUT.

ARB_geometry_shader4 spec 2.15.2:
"Linking will also fail [...] if the program object contains objects to form a
geometry shader, and the value of GEOMETRY_VERTICES_OUT_ARB is zero."

Reviewed-by: Paul Berry <stereotype441@gmail.com>

TODO: May not be necessary for GLSL 1.50 style geometry shaders.  Make a test to validate.
!UPSTREAM: Work around bug in AntTweakBar

AntTweakBar tries to use glGetString(GL_EXTENSIONS) when connected to
a core context (it should use glGetStringi()).  Go ahead and allow
this for the time being.
!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.