refactor: stage localcoord, StageVar, ModifyExplod

[potsmugen]

• Stage default values now automatically scale to the stage's localcoord
• More work done on making the "up" and "down" char velocity constants closer to what we know about those unused Mugen constants
• StageVar and ModifyStageVar now automatically do the necessary conversions to the localcoord of the character that uses them
• Adjusted ModifyExplod logic to rely less on some quirks of the Mugen engine. Fixes #2065