Improve idle performance and other enhancements

[bloopclick]

• Add dirty flag to only render after draw commands
• Modify oscilloscope draw command to suppress empty redraws
• Add ability to make a debug build from the command line
• Slightly optimize byte copies to SLIP parser

The dirty flag is an alternate solution for fixing the fx palette bug, but it can still add some value since it's a more flexible way to control when frames get drawn. For example, I've used it to suppress empty oscilloscope draws while M8 is not playing, which reduces the CPU usage significantly during idle.

Debug logging can now be enabled from the make command without editing the source code: make CFLAGS=-DDEBUG_MSG

[laamaa]

Seems to work well, thank you!! :)