Add HiDPI support for the Qt toolkit

[jwiggins]

This is the Qt half of #412 and #517. I haven't looked into how Wx does HiDPI display yet, but figuring out how to do it for Qt5 first will likely make adding Wx "easy".

Known broken:

• Component.use_backbuffer creates a low resolution buffer.
• OpenGL clipping is a bit off
• GraphicsContext.clip_to_rect uses absolute coordinates, so supporting HiDPI by changing the scale of a GC will break lots of code [but not all] which uses clip_to_rect GraphicsContext.clip_to_rect is not uniformly implemented with respect to the affine transformation across all kiva backends! (Celiagg's clip_to_rect() should respect the transform matrix #592)
• GraphicsContext.draw_marker_at_points in the kiva.agg backend will draw nothing if there's anything more than a translation applied to the transformation matrix. Covered by Allow scaling in agg's draw_marker_at_points #594

[corranwebster]

This looks generally fine in terms of implementation. I haven't taken it for a spin yet, so there may be some glitches out there still.

There are some edge-cases that I can see around what happens if the resolution changes while the window is live (either by the user changing the resolution, or moving between screens), but as-is should be no worse than what we have now. I think it can be supported in Qt if we wanted to go there, but worst case is that you get low-resolution images after moving from low-res to high-res; or you render at high-res when working with low-res.

This PR should include some tests: testing the Kiva GCs should be straightforward, but testing that the resolution is set correctly in CI is probably going to need a mock/patch of the appropriate Qt methods - I have no idea what headless boxes are going to report as screen resolution.

[corranwebster]

Just had a thought where there may be additional breakage: in save methods we probably need to adjust the dpi appropriately. Pillow will take the raw data just fine, but won't pass the scale information through to saved files.

[jwiggins]

I've added some modifications to the kiva drawing tests which exercise the pixel scaling code.

[jwiggins]

Shakes fist at OpenGL backend 😠

[jwiggins]

#593 added to track saved image DPI concerns.

[corranwebster]

Playing around with this a little, it seems to be working for the QPainter backend, but I think #390 might be impacting the drawing in the Agg backend - a little hard to tell as the number of pixels a line requires depends on the fractional position of the line - or alternatively it may be that we are snapping lines to coordinates somewhere (I have a vague recollection of such code).

[jwiggins]

#390 is definitely a factor for the AGG backends. They're definitely implemented incorrectly, but I can only imagine how to fix celiagg and not how to fix kiva.agg.

[jwiggins]

Thanks for the testing and feedback.