builtin cheat to help Extreme Hunting 2 boot

pvr: last naomi2 poly was ignored in some cases

Fix the disappearing moustache in wild riders
Fix other potential edge cases in pvr parsing

macos: fix size for image icons

wrungpo had wrong decryption key

Issue libretro/flycast#1108

Add volume slider in audio settings (flyinghead#329)

Use logarithmic volume scale

maple: better vmu I/O error handling

First checkpoint for r7

Put a note about the beta compile link in the README.md