fix blurry text on Apple retina display

[VitoVan]

This patch handles the text rendering on high-DPI screen for macOS.

I mark this PR Work-in-Progress, because I haven't fully tested this patch, here is the check list:

For MacOS

• no more blurry text on macOS with retina screen
• automatic font resizing when moving window between high-DPI screen and low-DPI screen
• don't break low-DPI screen rendering (when start from the low-DPI screen)

For other platforms

• does it break the rendering on Windows?
  It was broken by itself, this patch didn't make it worse.
  SDL2: Windows High-DPI rendering problem #801
• does it break the rendering on Linux?
  It was broken by itself, this patch didn't make it worse.
  SDL2: Linux High-DPI rendering problem #802

[VitoVan]

Current problem:

If :accelerated is not allowed in renderer flags, then the high-DPI problem could be very hard to solve.

Without that flag, SDL2 won't be high-DPI awareness, so we can't handle it.

But, it seems there are also some problems when we set that flag: #787

So, what's your thought?

I'm going to make this patch macOS only.

For other platform, let's solve it as another problem.

[VitoVan]

This is my first PR, I must have done many things inappropriate, so if you saw any, please don't hesitate to tell me.

[cxxxr]

Thank you very much for the great PR.