add support to software opengl backend

[vaguinerg]

{.passC: "-DGRAPHICS_API_OPENGL_11_SOFTWARE".}
asks for external/rlsw.h
needs to update raylib

[planetis-m]

Contribs welcome! Ask to update tools/wrapper/snippets/raylib_header.nim
and then follow the instructions in manual/update_guide.md from step 4 onwards (skipping updating raylib source).