Don't require glew 2.0

[ghost]

This is an extension of #36

The general changes added by glew 2.0 (aside from extensions that are not used in kitty) are:

Enhancements:

• Forward context support added
• OSMesa support added
• EGL support added
• MX support discontinued
• Improved cmake build support

The top three are OSX specific. If I change setup.py to only require 1.13.0 and implement the patch in my previous issue where the window hints are only applied for isosx it works perfectly fine on my system.

[kovidgoyal]

I'm not sure what the minimum required version of glew is, I do know that everything works with glew 2.0. And as mentioned on #36 those tests are not OS X specific. I prefer kitty fail with a clear and comprehensible error at build time rather than either not function correctly or fail with a cryptic error message at run time. Initially, I had no hard requirement on glew >= 2.0, but there was somebody for whom kitty failed at runtime with an older version of glew installed. And annoyingly it failed with the error

RuntimeError: OpenGL is missing the required ARB_copy_image extension

so it was not at all clear that an outdated version of glew was to blame.