This adds the glfwInitAllocator function for specifying a custom memory
allocator to use instead of the C runtime library.
The allocator is a struct of type GLFWallocator with fields
corresponding to malloc, realloc and free, while the internal API
corresponds to calloc, realloc and free.
Heap allocation calls are filtered before reaching the user-provided
functions, so deallocation of NULL and allocations of zero bytes are not
passed on, reallocating NULL is transformed into an allocation and
reallocating to size zero is transformed into deallocation.
The clearing of a new block to zero is performed by the internal
calloc-like function.
Closes#544.
Fixes#1628.
Closes#1947.
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.
