gallium/hud: add more options to customize HUD panes

gallium/hud: add more options to customize HUD panes

Extends the syntax of GALLIUM_HUD environment variable to:
- Add options to set the size and exact location of each pane.
- Add an option to limit the maximum allowed value of the X axis on a
  pane, clamping the graph down to not go above this value.
- Add an option to auto-adjust the value of the Y axis down to the
  highest value still visible on the graph.

v2:
- Make the patch simpler and smaller.
- With dynamic auto-adjusting on, adjust the Y axis once per pane
  update instead of updating once every several seconds.
- No longer mishandle pane height when having more than one graph per
  pane.