static inline GLenum to_gl_swizzle(int swizzle)
{
   switch (swizzle) {
   case PIPE_SWIZZLE_RED: return GL_RED;
   case PIPE_SWIZZLE_GREEN: return GL_GREEN;
   case PIPE_SWIZZLE_BLUE: return GL_BLUE;
   case PIPE_SWIZZLE_ALPHA: return GL_ALPHA;
   case PIPE_SWIZZLE_ZERO: return GL_ZERO;
   case PIPE_SWIZZLE_ONE: return GL_ONE;
   default:
      assert(0);
      return 0;
   }
}
static inline GLenum to_gl_swizzle(int swizzle)
{
   switch (swizzle) {
   case PIPE_SWIZZLE_RED: return GL_RED;
   case PIPE_SWIZZLE_GREEN: return GL_GREEN;
   case PIPE_SWIZZLE_BLUE: return GL_BLUE;
   case PIPE_SWIZZLE_ALPHA: return GL_ALPHA;
   case PIPE_SWIZZLE_ZERO: return GL_ZERO;
   case PIPE_SWIZZLE_ONE: return GL_ONE;
   default:
      assert(0);
      return 0;
   }
}