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.

docs/compile.dox

@@ -97,6 +97,17 @@ Once you have installed the necessary packages, move on to @ref
 compile_generate.
 
 
+@subsection compile_deps_osmesa Dependencies for Linux and OSMesa
+
+To compile GLFW for OSMesa, you need to install the OSMesa library and header
+packages.  For example, on Ubuntu and other distributions based on Debian
+GNU/Linux, you need to install the `libosmesa6-dev` package.  The OSMesa library
+is required at runtime for context creation and is loaded on demand.
+
+Once you have installed the necessary packages, move on to @ref
+compile_generate.
+
+
 @subsection compile_generate Generating build files with CMake
 
 Once you have all necessary dependencies it is time to generate the project
@@ -249,6 +260,7 @@ ramps and clipboard.  The options are:
  - `_GLFW_X11` to use the X Window System
  - `_GLFW_WAYLAND` to use the Wayland API (experimental and incomplete)
  - `_GLFW_MIR` to use the Mir API (experimental and incomplete)
+ - `_GLFW_OSMESA` to use the OSMesa API (headless and non-interactive)
 
 If you are building GLFW as a shared library / dynamic library / DLL then you
 must also define `_GLFW_BUILD_DLL`.  Otherwise, you must not define it.