We already have renderer.pixel_ratio, but its behavior is not so great when the screen's pixel_ratio is a fractional.

With this PR:

• renderer.pixel_scale is the factor between the internal texture and target physical size.
  • For values > 1 it represents the amount of ssaa.
  • Setting to 1 matches the internal texture size with the target resolution.
• renderer.pixel_ratio is the factor between the internal texture and target logical size.
  • It represents the total pixel_ratio.
• Setting either to None will chose a pixel_scale of either 1 or 2, depending on whether the screen looks like it's hiDPI.
  • The current code already intended to do this, but kindof failed for e.g. 125% OS scaling on Windows/Linux.

For reference, the behavior of OS display scaling

• MacOS
  • Can select among a few different "text sizes".
  • MacOS uses an internal framebuffer that is sized smaller for "large text" mode, and then rendered to screen.
  • In effect the pixel_ratio is always 2.0.
• Windows:
  • Select resolution, and then a scale (100%, 125%, 150%, 175%).
  • On a HiDPI display, can also select scales 200% and beyond.
  • The display scale percentage is directly reflected in the canvas.pixel_ratio. E.g. 225%-> 2.25
• Linux Wayland
  • Select resolution, and then a scale. Fractional scales are experimental.
  • Qt backend:
    • Scale 100% and 200% result in pixel ratios 1 and 2.
    • Fractional scales work too, e.g. pixel_ratio 1.25
  • Glfw backend:
    • Stuck to pixel_ratio 1.0 -> a bug in glfw (I checked that its not our wrapper).
• Linux X11
  • Select resolution, and then a scale. Fractional scales are experimental.
  • Need to restart IDE / pygfx to take effect.
  • Scale 100% and 200% result in pixel ratios 1 and 2.
  • Fractional scales don't work 150% -> 2.0, 250% -> 3.0
  • Same behavior for glfw and Qt.

What this means

• On MacOS the canvas.pixel_ratio is always 2.0, as far as I can see.
• Mac simply scales the whole rendered screen (it can do that because it can assume it has a Retina display).
• Windows and Linux work more or less the same.
• Users may have a pixel_ratio of 1.25 or 1.5 (i.e. if they use a large screen and place it further away).
• We can assume that with a pixel_ratio of 2.0+ we're dealing with a HiDPI display.

To summarize this PR in the context of the above; we use pixel_ratio>=2 to detect HiDPI displays and turn on SSAA automatically if its not, being careful to select either a factor of 1 or 2, because the ssaa.wgsl shader is optimized for these values.