Improves the conversion from RGB888 to RGB32 on platforms without SIMD
versions. This includes the fallback used on non-neon ARM devices.
Besides image conversion the routine is also used for decoding JPEG.
On x86 this version is within 0.7x of the speed of the SSSE3 version.
Change-Id: Id131994d7c3c4f879d89e80f9d6c435bb5535ed7
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>