Rename bpmem.copyMipMapStrideChannels to bpmem.copyDestStride

As far as I can tell, it has nothing to do with the mipmap/half_scale functionality, but does change based on the width of the destination texture (and the destination texture is half the width if half_scale is set). The comment that was there (which dates back to the initial megacommit) seems to not have accounted for the width aspect; it was first used as an actual stride in bbbe898839 (the first commit that used it at all).

Source/Core/VideoBackends/Software/TextureEncoder.cpp

@@ -231,7 +231,7 @@ static void SetSpans(int sBlkSize, int tBlkSize, s32* tSpan, s32* sBlkSpan, s32*
   *tBlkSpan = ((640 * tBlkSize) - alignedWidth) *
               readStride;  // bytes to advance src pointer after each row of blocks
 
-  *writeStride = bpmem.copyMipMapStrideChannels * 32;
+  *writeStride = bpmem.copyDestStride << 5;
 }
 
 #define ENCODE_LOOP_BLOCKS                                                                         \