context creation fails on X11 with Nvidia drivers if alpha_bits != 0

[glowcoil]

If a context is requested with alpha_bits != on X11 with Nvidia drivers, the following error is generated:

X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  151 (GLX)
  Minor opcode of failed request:  5 (X_GLXMakeCurrent)
  Serial number of failed request:  50
  Current serial number in output stream:  50

This can be worked around by requesting 0 alpha bits. Possible avenues for a long-term fix:

• more elaborate/thorough pixel format selection logic
• switching to EGL

This appears to be the same problem observed here: https://github.com/opentk/opentk/issues/108

[glowcoil]

Related: https://github.com/rust-windowing/glutin/issues/834