st/mesa: set stencil border color the same as intensity

This fixes some stencil border color tests on Vega and Raven chips.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/mesa/state_tracker/st_format.c

@@ -2545,6 +2545,8 @@ st_translate_color(const union gl_color_union *colorIn,
          out[0] = out[1] = out[2] = in[0];
          out[3] = in[3];
          break;
+      /* Stencil border is tricky on some hw. Help drivers a little here. */
+      case GL_STENCIL_INDEX:
       case GL_INTENSITY:
          out[0] = out[1] = out[2] = out[3] = in[0];
          break;