20210707 dev build

Changes since the last release:

• fixed main screen memory contention alignment (#54).
• fixed debugger single-stepping just index prefixes (#53).
• fixed instruction trace showing double registers when only half changed.
• fixed step-out feature in debugger.
• fixed debugger benchmark mode still processing interrupts.
• fixed cancelled line interrupt end event showing in debugger.
• added 'tryvrr' option (default=true) to try Variable Refresh Rate video.
• added Ctrl modifier to run more instructions using the KP4/5/6.