st/mesa: free the temporary bitmap/drawpix shader code

Fixes a per-shader memory leak when drawing glBitmaps, glDrawPixels
or glCopyPixels.

NOTE: This is a candidate for the 7.9 and 7.10 branches.

src/mesa/state_tracker/st_program.c

 #if FEATURE_drawpix
    if (key->bitmap) {
       /* glBitmap drawing */
-      struct gl_fragment_program *fp;
+      struct gl_fragment_program *fp; /* we free this temp program below */
 
       st_make_bitmap_fragment_program(st, &stfp->Base,
                                       &fp, &variant->bitmap_sampler);
    }
    else if (key->drawpixels) {
       /* glDrawPixels drawing */
-      struct gl_fragment_program *fp;
+      struct gl_fragment_program *fp; /* we free this temp program below */
 
       if (key->drawpixels_z || key->drawpixels_stencil) {
          fp = st_make_drawpix_z_stencil_program(st, key->drawpixels_z,
       debug_printf("\n");
    }
 
+#if FEATURE_drawpix
+   if (key->bitmap || key->drawpixels) {
+      /* Free the temporary program made above */
+      struct gl_fragment_program *fp = &stfp->Base;
+      _mesa_reference_fragprog(st->ctx, &fp, NULL);
+   }
+#endif
+
    return variant;
 }