blitbuffer.c: small optimisation by switching x/y loops (#799)
blitbuffer.c: small optimisation by switching x/y loops (#799)

Scanning y in the outer loop and x in the inner loop allows
scannning memory linearly and avoid CPU cache trashing.
(This gives a 25% performance boost on the emulator in portrait
mode, and might as well on Android.)
houqp/koreader-base,Frenzie/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,Frenzie/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,houqp/koreader-base,koreader/koreader-base,koreader/koreader-base,houqp/koreader-base