vrend: add support for stencil texturing.

This enables:
GL_ARB_stencil_texturing, GL_ARB_texture_stencil8.

src/vrend_formats.c

@@ -62,6 +62,7 @@ static struct vrend_format_table base_depth_formats[] =
     { VIRGL_FORMAT_Z32_FLOAT, GL_DEPTH_COMPONENT32F, GL_DEPTH_COMPONENT, GL_FLOAT },
     /* this is probably a separate format */
     { VIRGL_FORMAT_Z32_FLOAT_S8X24_UINT, GL_DEPTH32F_STENCIL8, GL_DEPTH_STENCIL, GL_FLOAT_32_UNSIGNED_INT_24_8_REV},
+    { VIRGL_FORMAT_X24S8_UINT, GL_STENCIL_INDEX8, GL_STENCIL_INDEX, GL_UNSIGNED_INT_24_8, 0 },
   };
 
 static struct vrend_format_table base_la_formats[] = {