mesa: add support for GL_OES_EGL_image_external

This is an OpenGL ES specific extension.  External textures are textures that
may be sampled from, but not be updated (no glTexSubImage* and etc.).  The
image data are taken from an EGLImage.

Reviewed-by: Brian Paul <brianp@vmware.com>
Acked-by: Jakob Bornecrantz <jakob@vmware.com>

src/mesa/program/ir_to_mesa.cpp

@@ -2273,6 +2273,9 @@ ir_to_mesa_visitor::visit(ir_texture *ir)
    case GLSL_SAMPLER_DIM_BUF:
       assert(!"FINISHME: Implement ARB_texture_buffer_object");
       break;
+   case GLSL_SAMPLER_DIM_EXTERNAL:
+      inst->tex_target = TEXTURE_EXTERNAL_INDEX;
+      break;
    default:
       assert(!"Should not get here.");
    }