Fixed deflated output and handling of palette images

This release improves the efficiency of the output data size by using the correct zlib flush options and disabling pre-filtering of palette images.

Additional fixes needed

The FLATE output still wasn't being flushed correctly until I added this fix. Now the compression is good and the output is properly written when larger than the output buffer size.

Fixed compression

I had used an incorrect option for zlib to compress the data with "sync" which resets the compression engine after each line. Now it compresses the output much smaller. I also added some missing C functions.

Added RGB565 support

This release adds a new method to make it easier to compress RGB565 images (which aren't natively supported by PNG). The encodeBegin() method will specify if you want to create a truecolor or grayscale image, then use addRGB565Line() to pass RGB565 pixels to the encoder.

initial release

An Arduino/embedded library to compress images to PNG format. Designed to use a 'practical' amount of RAM (45K) for microcontrollers.