gallium/util: implement layered framebuffer clear in u_blitter

All bound layers (from first_layer to last_layer) should be cleared.

This uses a vertex shader which outputs gl_Layer = gl_InstanceID, so each
instance goes to a different layer. By rendering a quad and setting
the instance count to the number of layers, it will trivially clear all
layers.

This requires AMD_vertex_shader_layer (or PIPE_CAP_TGSI_VS_LAYER), which only
radeonsi supports at the moment. r600 could do this too. Standard DX11
hardware will have to use a geometry shader though, which has higher overhead.

src/gallium/drivers/r300/r300_blit.c

@@ -365,9 +365,7 @@ static void r300_clear(struct pipe_context* pipe,
     if (buffers) {
         /* Clear using the blitter. */
         r300_blitter_begin(r300, R300_CLEAR);
-        util_blitter_clear(r300->blitter,
-                           width,
-                           height,
+        util_blitter_clear(r300->blitter, width, height, 1,
                            buffers, color, depth, stencil);
         r300_blitter_end(r300);
     } else if (r300->zmask_clear.dirty ||