OSMesa: Cleanup

Fixes formatting, semantics and documentation.  Adds
glfwGetOSMesaContext.  Adds support for OSMesa context attributes.
Updates changelog and credits.  Adds license and copyright headers.
Removes superfluous code (the shared code provides many conveniences).
Removes loading of unused OSMesa functions.  Removes empty platform
structs.  Fixes version string format.  Removes build dependency on
the OSMesa header and library (only the library is needed and only at
runtime).

Closes #850.

README.md

@@ -104,6 +104,7 @@ information on what to include when reporting a bug.
   scancodes for keys (#830)
 - Added `glfwSetWindowMaximizeCallback` and `GLFWwindowmaximizefun` for
   receiving window maximization events (#778)
+- Added headless [OSMesa](http://mesa3d.org/osmesa.html) backend (#281,#850)
 - Added definition of `GLAPIENTRY` to public header
 - Bugfix: Calling `glfwMaximizeWindow` on a full screen window was not ignored
 - Bugfix: `GLFW_INCLUDE_VULKAN` could not be combined with the corresponding
@@ -155,6 +156,7 @@ skills.
  - Lambert Clara
  - Andrew Corrigan
  - Noel Cower
+ - Jason Daly
  - Jarrod Davis
  - Olivier Delannoy
  - Paul R. Deppe