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Simplify mingle_2bpp_planes

Fixes #891

--- a/tools/bpp2png.c

+++ b/tools/bpp2png.c

@@ -70,8 +70,8 @@

 	for (long i = 0; i < size; i += 2) {

 		// Interleave aka "mingle" bits

 		// <https://graphics.stanford.edu/~seander/bithacks.html#Interleave64bitOps>

-#define EXPAND_PLANE(b) (((b) * 0x0101010101010101ULL & 0x8040201008040201ULL) * 0x0102040810204081ULL)

-		uint16_t r = ((EXPAND_PLANE(bpp_data[i]) >> 49) & 0x5555) | ((EXPAND_PLANE(bpp_data[i + 1]) >> 48) & 0xAAAA);

+#define EXPAND_PLANE(b) (((((b) * 0x0101010101010101ULL & 0x8040201008040201ULL) * 0x0102040810204081ULL) >> 48) & 0xAAAA)

+		uint16_t r = (EXPAND_PLANE(bpp_data[i]) >> 1) | EXPAND_PLANE(bpp_data[i + 1]);

 		bpp_data[i] = r >> 8;

 		bpp_data[i + 1] = r & 0xff;

 	}