## Problem
OpenGOAL uses OpenGL 4.3.
Apple stopped upgrading OpenGL after 4.1, so the way OpenGOAL currently
works will never be playable on Macs using OpenGL.
## Solution
Luckily, downgrading to OpenGL 4.1 is not a huge change (at least it
doesn't seem like it to my untrained eyes).
## Changes
* set hints for OpenGL 4.1 instead of 4.3 for __APPLE__
* skip the OpenGL debugging callback setup for macOS (requires 4.3)
* bump down the version string for all shaders
* stop using the `binding` layout qualifier in shader code
* move the `flat` qualifier first (not sure if this is a 4.1 thing or
just Macs being more strict)
* don't mix signed and unsigned ints in shaders (not sure if this is a
4.1 thing or just Macs being more strict)
* add some hacky CPP to the Shader constructor for binding texture units
and bones buffers based on variable names in the shader code
## Results
