Support gamma correction of text on GL

Support gamma correction of text on GL

If the SRGB framebuffer extension in GL is available, we can support
gamma correction of text with a gamma of 2.1. On Mac this is
sufficient for gamma correcting subpixel antialiased text. Gray
antialiasing should not be gamma corrected on Mac.

On Windows, the user can potentially set the gamma value to anything
between 1.0 and 2.2 (or something like that). We support anything
that resembles 1.0 closely enough by pushing the text out without
any correction (like before). We also support anything that resembles
2.1 (the gamma hardcoded in GL's SRGB extension) by turning on the
extension before blending the text. In between the two, we'll use
gray antialiasing to avoid differing too much from the raster engine
(which is our reference in this.)

For gray antialiasing on Windows, we use a constant gamma of 2.3 which
has been determined by experimentation. Since this is close enough to
2.1 we do gamma correction with SRGB extension.

The distance limit of 0.2 is determined by some experimentation.

Reviewed-by: Samuel
(cherry picked from commit 79ba7cceca5e4029876ace2121edd25b08ae14ce)