Add new decode members for alpha channel info

Add three new members in mfxInfoMFX (AlphaChannelExist, LosslessAlpha,
PreMultipliedAlpha) that the decoder populates during
MFXVideoDECODE_DecodeHeader. They allow applications to discover, prior
to allocating output surfaces, whether the input bitstream carries an
alpha channel, whether that alpha is encoded losslessly, and whether
the alpha has been pre-multiplied with the video channels.

The new bitfields are packed into reservedByte0 to preserve struct
layout and ABI. Static ABI offset/width assertions and change-log
entries are added accordingly.

Author: ChunjuanLiu

api/vpl/mfxstructures.h

@@ -1042,7 +1042,13 @@ typedef struct {
             struct {
                 /*! Output ScalingRatio Cap list, it's bit-wise and will be updated by MFXVideoDECODE_DecodeHeader, see OutputScalingRatioCaps enumerator for the definition. */
                 mfxU16  OutputScalingRatioCaps : 8;
-                mfxU16  reservedByte0 : 8;
+                /*! Set by MFXVideoDECODE_DecodeHeader. Nonzero value indicates that the input bitstream carries an alpha channel. */
+                mfxU16  AlphaChannelExist     : 1;
+                /*! Set by MFXVideoDECODE_DecodeHeader. Nonzero value indicates that the alpha channel is encoded losslessly. Valid only when AlphaChannelExist is nonzero. */
+                mfxU16  LosslessAlpha         : 1;
+                /*! Set by MFXVideoDECODE_DecodeHeader. Nonzero value indicates that the video channels have been pre-multiplied with alpha prior to encoding. */
+                mfxU16  PreMultipliedAlpha    : 1;
+                mfxU16  reservedByte0         : 5;
             };
             mfxU16  reserved2[1];
 #else