pixfmt: Use enum assignment for aliases

This way, Doxygen is happier as aliases are now grouped together, and
it never handled #define's in an enum well in the first place.

Y400A already exists as an enum assignment.

libavutil/pixfmt.h

@@ -77,7 +77,7 @@ enum AVPixelFormat {
 #if FF_API_XVMC
     AV_PIX_FMT_XVMC_MPEG2_MC,///< XVideo Motion Acceleration via common packet passing
     AV_PIX_FMT_XVMC_MPEG2_IDCT,
-#define AV_PIX_FMT_XVMC AV_PIX_FMT_XVMC_MPEG2_IDCT
+    AV_PIX_FMT_XVMC = AV_PIX_FMT_XVMC_MPEG2_IDCT,
 #endif /* FF_API_XVMC */
     AV_PIX_FMT_UYVY422,   ///< packed YUV 4:2:2, 16bpp, Cb Y0 Cr Y1
     AV_PIX_FMT_UYYVYY411, ///< packed YUV 4:1:1, 12bpp, Cb Y0 Y1 Cr Y2 Y3
@@ -178,6 +178,7 @@ enum AVPixelFormat {
     AV_PIX_FMT_YUV422P9LE, ///< planar YUV 4:2:2, 18bpp, (1 Cr & Cb sample per 2x1 Y samples), little-endian
     AV_PIX_FMT_VDA_VLD,    ///< hardware decoding through VDA
     AV_PIX_FMT_GBRP,      ///< planar GBR 4:4:4 24bpp
+    AV_PIX_FMT_GBR24P = AV_PIX_FMT_GBRP, // alias for #AV_PIX_FMT_GBRP
     AV_PIX_FMT_GBRP9BE,   ///< planar GBR 4:4:4 27bpp, big-endian
     AV_PIX_FMT_GBRP9LE,   ///< planar GBR 4:4:4 27bpp, little-endian
     AV_PIX_FMT_GBRP10BE,  ///< planar GBR 4:4:4 30bpp, big-endian
@@ -308,9 +309,6 @@ enum AVPixelFormat {
     AV_PIX_FMT_NB,        ///< number of pixel formats, DO NOT USE THIS if you want to link with shared libav* because the number of formats might differ between versions
 };
 
-#define AV_PIX_FMT_Y400A AV_PIX_FMT_GRAY8A
-#define AV_PIX_FMT_GBR24P AV_PIX_FMT_GBRP
-
 #if AV_HAVE_BIGENDIAN
 #   define AV_PIX_FMT_NE(be, le) AV_PIX_FMT_##be
 #else