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_cx_iface.c b/vp8/vp8_cx_iface.c
index e129ec9..737e627 100644
--- a/vp8/vp8_cx_iface.c
+++ b/vp8/vp8_cx_iface.c
@@ -211,10 +211,10 @@
 {
     switch (img->fmt)
     {
-    case IMG_FMT_YV12:
-    case IMG_FMT_I420:
-    case IMG_FMT_VPXI420:
-    case IMG_FMT_VPXYV12:
+    case VPX_IMG_FMT_YV12:
+    case VPX_IMG_FMT_I420:
+    case VPX_IMG_FMT_VPXI420:
+    case VPX_IMG_FMT_VPXYV12:
         break;
     default:
         ERROR("Invalid image format. Only YV12 and I420 images are supported");
@@ -549,7 +549,7 @@
     yv12->uv_stride = img->stride[PLANE_U];
 
     yv12->border  = (img->stride[PLANE_Y] - img->w) / 2;
-    yv12->clrtype = (img->fmt == IMG_FMT_VPXI420 || img->fmt == IMG_FMT_VPXYV12); //REG_YUV = 0
+    yv12->clrtype = (img->fmt == VPX_IMG_FMT_VPXI420 || img->fmt == VPX_IMG_FMT_VPXYV12); //REG_YUV = 0
     return res;
 }
 
@@ -846,7 +846,7 @@
     {
 
         /*
-        vpx_img_wrap(&ctx->preview_img, IMG_FMT_YV12,
+        vpx_img_wrap(&ctx->preview_img, VPX_IMG_FMT_YV12,
             sd.y_width + 2*VP8BORDERINPIXELS,
             sd.y_height + 2*VP8BORDERINPIXELS,
             1,
@@ -862,9 +862,9 @@
         ctx->preview_img.planes[PLANE_V] = sd.v_buffer;
 
         if (sd.clrtype == REG_YUV)
-            ctx->preview_img.fmt = IMG_FMT_I420;
+            ctx->preview_img.fmt = VPX_IMG_FMT_I420;
         else
-            ctx->preview_img.fmt = IMG_FMT_VPXI420;
+            ctx->preview_img.fmt = VPX_IMG_FMT_VPXI420;
 
         ctx->preview_img.x_chroma_shift = 1;
         ctx->preview_img.y_chroma_shift = 1;