Update: bugfixes and minor changes

What's new:

• New target sprite by Tibor.
• Inactive "future" targets no longer show at a low opacity.
• Slightly lower gravity and bigger player colliders for (hopefully) improved head stop detection.
• Bugfix: after all targets (diamonds) were picked up, the scoreboard showed up but the game did not advance to the next level after 10 seconds, only if Shift+L was pressed.
• Bugfix: when a player reached the land while wall riding, the animation did not reset to "standing".
• Bugfix (regression): mines did not kill NPC enemies.
• Added mobile (Android/iOS) detection and on-screen controls for the sole purpose of testing features on mobile. On-screen controls can be toggled with Shift+T.