Limits bitdepth and channels, specifies compression

Limits bitdepth to 8 and channels to 3 (no alpha).
It also specifies to use Run Length Encryption, which is
super quick, and effective for compressing desktop graphics.

This is necessary for SGI format to actualle use RLE, otherwise
it just went to fallback uncompressed.

    src/SConscript
    src/ScreenCapture.cpp
    src/main.cpp