Inclusion of internal headers is already both centralized and follows
strict rules. Inclusion guards are both an unneccessary maintenance
burden and may hide inclusion order bugs.
PFN_FunctionName is more readable than SCREAMSOUP_T.
Context creation API function typedefs are kept as-is where the original
header provided them, for compatibility and familiarity reasons.
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.
