vpx_image: add VPX_ prefix to PLANE_*

The PLANE_{PACKED,Y,U,V,ALPHA} macros should be renamed to be within the
VPX_ namespace.

Fixes #27
diff --git a/vp8/vp8_cx_iface.c b/vp8/vp8_cx_iface.c
index b45b212..5883abd 100644
--- a/vp8/vp8_cx_iface.c
+++ b/vp8/vp8_cx_iface.c
@@ -536,19 +536,19 @@
                                        YV12_BUFFER_CONFIG  *yv12)
 {
     vpx_codec_err_t        res = VPX_CODEC_OK;
-    yv12->y_buffer = img->planes[PLANE_Y];
-    yv12->u_buffer = img->planes[PLANE_U];
-    yv12->v_buffer = img->planes[PLANE_V];
+    yv12->y_buffer = img->planes[VPX_PLANE_Y];
+    yv12->u_buffer = img->planes[VPX_PLANE_U];
+    yv12->v_buffer = img->planes[VPX_PLANE_V];
 
     yv12->y_width  = img->d_w;
     yv12->y_height = img->d_h;
     yv12->uv_width = (1 + yv12->y_width) / 2;
     yv12->uv_height = (1 + yv12->y_height) / 2;
 
-    yv12->y_stride = img->stride[PLANE_Y];
-    yv12->uv_stride = img->stride[PLANE_U];
+    yv12->y_stride = img->stride[VPX_PLANE_Y];
+    yv12->uv_stride = img->stride[VPX_PLANE_U];
 
-    yv12->border  = (img->stride[PLANE_Y] - img->w) / 2;
+    yv12->border  = (img->stride[VPX_PLANE_Y] - img->w) / 2;
     yv12->clrtype = (img->fmt == VPX_IMG_FMT_VPXI420 || img->fmt == VPX_IMG_FMT_VPXYV12); //REG_YUV = 0
     return res;
 }
@@ -857,9 +857,9 @@
             */
 
         ctx->preview_img.bps = 12;
-        ctx->preview_img.planes[PLANE_Y] = sd.y_buffer;
-        ctx->preview_img.planes[PLANE_U] = sd.u_buffer;
-        ctx->preview_img.planes[PLANE_V] = sd.v_buffer;
+        ctx->preview_img.planes[VPX_PLANE_Y] = sd.y_buffer;
+        ctx->preview_img.planes[VPX_PLANE_U] = sd.u_buffer;
+        ctx->preview_img.planes[VPX_PLANE_V] = sd.v_buffer;
 
         if (sd.clrtype == REG_YUV)
             ctx->preview_img.fmt = VPX_IMG_FMT_I420;
@@ -871,9 +871,9 @@
 
         ctx->preview_img.d_w = ctx->cfg.g_w;
         ctx->preview_img.d_h = ctx->cfg.g_h;
-        ctx->preview_img.stride[PLANE_Y] = sd.y_stride;
-        ctx->preview_img.stride[PLANE_U] = sd.uv_stride;
-        ctx->preview_img.stride[PLANE_V] = sd.uv_stride;
+        ctx->preview_img.stride[VPX_PLANE_Y] = sd.y_stride;
+        ctx->preview_img.stride[VPX_PLANE_U] = sd.uv_stride;
+        ctx->preview_img.stride[VPX_PLANE_V] = sd.uv_stride;
         ctx->preview_img.w   = sd.y_width;
         ctx->preview_img.h   = sd.y_height;