12-bit JPEG support

[dcommander]

This would be used in conjunction with 30-bit (10/10/10/2 or 2/10/10/10) visuals, support for which will be available in the next major releases of VirtualGL and TurboVNC. This feature would allow the TurboVNC Server to send 30-bit images to a 30-bit-enabled client without downsampling.

Issues/comments:

• libjpeg-turbo is not currently SIMD-accelerated when used with 12-bit coefficients. Refer to Performance improvements (SIMD extensions) for 12-bit JPEGs libjpeg-turbo/libjpeg-turbo#199.
• It is unclear whether Java supports 30-bit visuals or how to use deep color from a Win32 native program without using OpenGL. (Further research required.)
• It would be necessary to have two different versions of the TurboJPEG API library, one for 8-bit and one for 12-bit, which would complicate Java Web Start deployment.
• The Tight encoder/decoder would have to be modified to support encoding from/decoding to 30-bit pixels. It would probably be best to introduce a new encoding or sub-encoding type for this, since the viewer will need to know whether or not to use the 12-bit decoder. Otherwise, it would have to somehow probe the JPEG image to figure out which decoder to use, but this would create a backward incompatibility with older viewers that don't support 12-bit JPEGs.