add overwrite checkbox for alpha ramps accuracy

git-svn-id: svn://svn.code.sf.net/p/vpinball/code/trunk@930 14438d79-0ffd-4333-8ae7-ea2bdd901387
Merge branch 'master' into fixes
Merge branch 'fixes' into dev

Conflicts:
	pintable.cpp
	vbatest_eng.rc