Implement EXT_discard_framebuffer in D3D11 renderer

Change-Id: I52bcf0cfb1aa123e085a35730fdefb006b617c3c
Reviewed-on: https://chromium-review.googlesource.com/269232
Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/gl/FramebufferGL.cpp b/src/libANGLE/renderer/gl/FramebufferGL.cpp
index c3f7890..732275b 100644
--- a/src/libANGLE/renderer/gl/FramebufferGL.cpp
+++ b/src/libANGLE/renderer/gl/FramebufferGL.cpp
@@ -152,6 +152,12 @@
     }
 }
 
+gl::Error FramebufferGL::discard(size_t count, const GLenum *attachments)
+{
+    UNIMPLEMENTED();
+    return gl::Error(GL_INVALID_OPERATION);
+}
+
 gl::Error FramebufferGL::invalidate(size_t count, const GLenum *attachments)
 {
     // Since this function is just a hint and not available until OpenGL 4.3, only call it if it is available.