This uses the existing pack/unpack testing infrasturcture for GLSL ES
3.0 and adds support for testing pack/unpack 4x8 operations.

Generate the following test files:
	{const,vs,fs}-{pack,unpack}{Snorm,Unorm}{2x16,4x8}.shader_test
	{const,vs,fs}-{pack,unpack}Half2x16.shader_test

Note that division is performed by multiplication by the reciprocal, and
1/255 falls almost exactly in between two representable float32 values,
leading to bad rounding errors. 1/127, 1/32767, and 1/65535 do not have
this behavior.

As a result, the {fs,vs}-unpackUnorm4x8 tests currently fail. the const-
version passes, since the divisions are calculated by the CPU. A
follow-on patch will allow some tolerance in precision.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>