Intel shader integer functions2 (#331)

* Initial version of GL_INTEL_shader_integer_functions2

* INTEL_shader_integer_functions2: Add interactions with EXT_shader_explicit_arithmetic_types

* INTEL_shader_integer_functions2: Resolve last two issues

The name is pretty well set in stone now, and there's little to no value
in providing 64-bit arithmetic functions.

* INTEL_shader_integer_functions2: Add extension number

v2: Run 'make' as instructed by README.adoc.

* INTEL_shader_integer_functions2: Fix a bunch of typos noticed by @cmarcelo
4 files changed