glsl: add support for GL_OES_EGL_image_external

This extension introduces a new sampler type: samplerExternalOES.
texture2D (and texture2DProj) can be used to do a texture look up in an
external texture.

Reviewed-by: Brian Paul <brianp@vmware.com>
Acked-by: Jakob Bornecrantz <jakob@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/glsl_lexer.ll

@@ -296,6 +296,13 @@ usamplerCube		KEYWORD(130, 130, USAMPLERCUBE);
 usampler1DArray		KEYWORD(130, 130, USAMPLER1DARRAY);
 usampler2DArray		KEYWORD(130, 130, USAMPLER2DARRAY);
 
+samplerExternalOES	{
+			  if (yyextra->OES_EGL_image_external_enable)
+			     return SAMPLEREXTERNALOES;
+			  else
+			     return IDENTIFIER;
+			}
+
 
 struct		return STRUCT;
 void		return VOID_TOK;