mesa: call DrawBufferAllocate driver hook in update_framebuffer for windows-system FB

When draw buffers are changed on a bound framebuffer, DrawBufferAllocate()
hook should be called. However, it is missing in update_framebuffer with
window-system framebuffer, in which FB's draw buffer state should match
context state, potentially resulting in a change.

Note: This is needed because gallium delays creating the front buffer,
      i965 works fine without this change.

V2 (Timothy Arceri):
 - Rebased on merged/simplified DrawBuffer driver function
 - Move DrawBuffer call outside fb->ColorDrawBuffer[0] !=
   ctx->Color.DrawBuffer[0] check to make piglit pass.

v3 (Timothy Arceri):
 - Call new DrawBuffaerAllocate() driver function.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de> (v2)
Reviewed-by: Brian Paul <brianp@vmware.com> (v2)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99116

src/mesa/main/framebuffer.c

@@ -617,6 +617,12 @@ update_framebuffer(struct gl_context *ctx, struct gl_framebuffer *fb)
          _mesa_drawbuffers(ctx, fb, ctx->Const.MaxDrawBuffers,
                            ctx->Color.DrawBuffer, NULL);
       }
+
+      /* Call device driver function if fb is the bound draw buffer. */
+      if (fb == ctx->DrawBuffer) {
+         if (ctx->Driver.DrawBufferAllocate)
+            ctx->Driver.DrawBufferAllocate(ctx);
+      }
    }
    else {
       /* This is a user-created framebuffer.