Rename FilterMode for compatibility with libyuv

Renames FilterMode to FilterModeEnum for compatibility
with google3 libyuv

Change-Id: Iec845e6a6aeac4171790c87ce3dd9000ab840a36

vpxdec.c

diff --git a/vpxdec.c b/vpxdec.c
index 4c37234..6356961 100644
--- a/vpxdec.c
+++ b/vpxdec.c
@@ -119,7 +119,7 @@
 #endif
 
 static int vpx_image_scale(vpx_image_t *src, vpx_image_t *dst,
-                           FilterMode mode) {
+                           FilterModeEnum mode) {
   assert(src->fmt == VPX_IMG_FMT_I420);
   assert(dst->fmt == VPX_IMG_FMT_I420);
   return I420Scale(src->planes[VPX_PLANE_Y], src->stride[VPX_PLANE_Y],