glsl: Implement MESA_shader_integer_mix extension.

Because why doesn't GLSL allow you to do this already?

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

docs/specs/MESA_shader_integer_mix.spec

@@ -0,0 +1,135 @@
+Name
+
+    MESA_shader_integer_mix
+
+Name Strings
+
+    GL_MESA_shader_integer_mix
+
+Contact
+
+    Matt Turner (matt.turner 'at' intel.com)
+
+Contributors
+
+    Matt Turner, Intel
+    Ian Romanick, Intel
+
+Status
+
+    Shipping
+
+Version
+
+    Last Modified Date:         09/09/2013
+    Author Revision:            5
+
+Number
+
+    
+
+Dependencies
+
+    OpenGL 3.0 or OpenGL ES 3.0 is required. This extension interacts with
+    GL_ARB_ES3_compatibility.
+
+    This extension is written against the OpenGL 4.4 (core) specification
+    and the GLSL 4.40 specification.
+
+Overview
+
+    GLSL 1.30 (and GLSL ES 3.00) expanded the mix() built-in function to
+    operate on a boolean third argument that does not interpolate but
+    selects. This extension extends mix() to select between int, uint,
+    and bool components.
+
+New Procedures and Functions
+
+    None.
+
+New Tokens
+
+    None.
+
+Additions to Chapter 8 of the GLSL 4.40 Specification (Built-in Functions)
+
+    Modify Section 8.3, Common Functions
+
+    Additions to the table listing common built-in functions:
+
+      Syntax                       Description
+      ---------------------------  --------------------------------------------------
+      genIType mix(genIType x,     Selects which vector each returned component comes
+                   genIType y,     from. For a component of a that is false, the
+                   genBType a)     corresponding component of x is returned. For a
+      genUType mix(genUType x,     component of a that is true, the corresponding
+                   genUType y,     component of y is returned.
+                   genBType a)
+      genBType mix(genBType x,
+                   genBType y,
+                   genBType a)
+
+Additions to the AGL/GLX/WGL Specifications
+
+    None.
+
+Modifications to The OpenGL Shading Language Specification, Version 4.40
+
+    Including the following line in a shader can be used to control the
+    language features described in this extension:
+
+      #extension GL_MESA_shader_integer_mix : <behavior>
+
+    where <behavior> is as specified in section 3.3.
+
+    New preprocessor #defines are added to the OpenGL Shading Language:
+
+      #define GL_MESA_shader_integer_mix        1
+
+Interactions with ARB_ES3_compatibility
+
+    On desktop implementations that support ARB_ES3_compatibility,
+    GL_MESA_shader_integer_mix can be enabled (and the new functions
+    used) in shaders declared with '#version 300 es'.
+
+GLX Protocol
+
+    None.
+
+Errors
+
+    None.
+
+New State
+
+    None.
+
+New Implementation Dependent State
+
+    None.
+
+Issues
+
+    1) Should we allow linear interpolation of integers via a non-boolean
+       third component?
+
+    RESOLVED: No.
+
+    2) Should we allow mix() to select between boolean components?
+
+    RESOLVED: Yes. Implementing the same functionality using casts would be
+    possible but ugly.
+
+Revision History
+
+    Rev.    Date      Author    Changes
+    ----  --------    --------  ---------------------------------------------
+      5   09/09/2013  idr       Add ARB_ES3_compatibility interaction.
+
+      4   09/06/2013  mattst88  Allow extension on OpenGL ES 3.0.
+
+      3   08/28/2013  mattst88  Add #extension/#define changes.
+
+      2   08/26/2013  mattst88  Change vendor prefix to MESA. Add mix() that
+                                selects between boolean components.
+      1   08/26/2013  mattst88  Initial revision