Add missing return statement

Update to work with latest Hare stdlib and Jinja

Changes include mostly:
 - *void -> *opaque
 - char -> types::c::char
 - jinja::contextfilter -> jinja::pass_context
 - avoid duplicate values (due to aliases) in switch statements

Disclaimer: I have no idea what I am doing :) but I did get the
vulkan-hello-world example to work and also the starfield GL example.

update requirements.txt due to https://github.com/mkdocs/mkdocs/issues/2799

hare: fix gl callback types

hare: add some vulkan types and the protection function

hare: add vulkan definition generation support

remove prefix from constants

move cstr conversion to get_proc_address() functions, halving code size

use *const char instead *const u8 for C strings

use string literals with explicit \0 as C strings

add more types

make sure len parameter is renamed

update hare directory structure

delete other type files

add support for user-supplied loaders

change glx to egl

add functional hare bindings

fix some gl types and add loader sketch

add hare generator sketch

example: defines GLAD_GL_IMPLEMENTATION in GLFW example (#464)