Address review comments #2, #3, and #10:
- Extract duplicated bayer4x4 matrix, applyBayerDither4Level(), and
drawPixelWithRenderMode() into shared DitherUtils.h
- Extract duplicated PixelCache struct into shared PixelCache.h so both
JPEG and PNG decoders use the same implementation
- Add MAX_CACHE_BYTES (256KB) size limit to PixelCache::allocate() to
proactively guard against oversized allocations on embedded targets
