glsl: Fix error checking on "flat" keyword to match GLSL ES 3.00, GLSL 1.50.

All of the GLSL specs from GLSL 1.30 (and GLSL ES 3.00) onward contain
language requiring certain integer variables to be declared with the
"flat" keyword, but they differ in exactly *when* the rule is
enforced:

(a) GLSL 1.30 and 1.40 say that vertex shader outputs having integral
type must be declared as "flat".  There is no restriction on fragment
shader inputs.

(b) GLSL 1.50 through 4.30 say that fragment shader inputs having
integral type must be declared as "flat".  There is no restriction on
vertex shader outputs.

(c) GLSL ES 3.00 says that both vertex shader outputs and fragment
shader inputs having integral type must be declared as "flat".

Previously, Mesa's behaviour was consistent with (a).  This patch
makes it consistent with (b) when compiling desktop shaders, and (c)
when compiling ES shaders.

Rationale for desktop shaders: once we add geometry shaders, (b) really
seems like the right choice, because it requires "flat" in just the
situations where it matters.  Since we may want to extend geometry
shader support back before GLSL 1.50 (via ARB_geometry_shader4), it
seems sensible to apply this rule to all GLSL versions.  Also, this
matches the behaviour of the nVidia proprietary driver for Linux, and
the expectations of Intel's oglconform test suite.

Rationale for ES shaders: since the behaviour specified in GLSL ES
3.00 matches neither pre-GLSL-1.50 nor post-GLSL-1.50 behaviour, it
seems likely that this was a deliberate choice on the part of the GLES
folks to be more restrictive.  Also, the argument in favor of (b)
doesn't apply to GLES, since it doesn't support geometry shaders at
all.

Some discussion about this has already happened on the Mesa-dev list.
See:

http://lists.freedesktop.org/archives/mesa-dev/2013-February/034199.html