Put img_fmt in the vpx namespace

Avoid an potential name clashes and match other external types.
s/IMG_FMT/VPX_$&/g
s/img_fmt/vpx_$&/g

Change-Id: Ia7ad5bbb6424416b37e71e5f5eb1eca31c3c707f
diff --git a/vp8/vp8_dx_iface.c b/vp8/vp8_dx_iface.c
index 3e6cdf4..8aaa52c 100644
--- a/vp8/vp8_dx_iface.c
+++ b/vp8/vp8_dx_iface.c
@@ -434,7 +434,7 @@
             unsigned int a_w = (sd.y_width + 15) & ~15;
             unsigned int a_h = (sd.y_height + 15) & ~15;
 
-            vpx_img_wrap(&ctx->img, IMG_FMT_I420,
+            vpx_img_wrap(&ctx->img, VPX_IMG_FMT_I420,
                          a_w + 2 * VP8BORDERINPIXELS,
                          a_h + 2 * VP8BORDERINPIXELS,
                          1,
@@ -570,7 +570,7 @@
     yv12->uv_stride = img->stride[PLANE_U];
 
     yv12->border  = (img->stride[PLANE_Y] - img->d_w) / 2;
-    yv12->clrtype = (img->fmt == IMG_FMT_VPXI420 || img->fmt == IMG_FMT_VPXYV12);
+    yv12->clrtype = (img->fmt == VPX_IMG_FMT_VPXI420 || img->fmt == VPX_IMG_FMT_VPXYV12);
 
     return res;
 }