Assorted fixes and improvements, after watching people playing (and complaining) :

• Added ability to (un)equip player upgrades.  Lots of people have issues with 'agility' upgrade -- it changes player character handling, permanently. This breaks the game for certain people. So now any (unlocked) upgrade can be enabled / disabled.
• Improved post-effect, for better feel of speed.
• Increased resolution of all environment textures, tweaked visuals of certain levels.
• Fixed several rendering bugs, to resolve issues affecting (mostly) integrated GPUs.
• Streamlined player controls.  Default scheme for gamepad now is left-stick for movement and right-stick for dodge, with ability to swap sticks. Dodge (on keyboard) is now only SHIFT+direction (or CTRL+direction), no more double-tap option -- it only caused troubles.
• Improved in-game HUD usability, yet again.