DGen 1.31

It's been a while! Here's the changelog since v1.30.

- Joystick axes can now be configured for controllers with more than two. They can also be reversed.
- Implemented command "calibrate_js" to configure joystick buttons interactively.
- Fixed a bug in the debugger prompt handler.
- Implemented FM2612 debugging support.
- Refactored SDL keyboard handling.
- Removed Linux-specific joystick code which no one uses (not even in Linux, where the SDL version has always been preferred).
- Implemented the ability to bind arbitrary commands to joystick buttons. This makes DGen fully controllable from a joystick/joypad.
- Implemented proper sprite masking. This fixes annoying priority glitches in a number of games.
- Fixed half-submitted commands support in the VDP. This finally fixes the mangled EA logo.
- Some code refactoring and documentation.
- Improved region settings. The single "region" variable can be used to switch interactively from one region settings to another (identifier, resolution, PAL/NTSC mode and frame rate).
- Improved region auto detection with the ability to specify a preference order ("str_region_order" variable).
- Fixed other miscellaneous bugs (NASM on non-x86 targets, uninitialized variables, other warnings and errors).