[LIBNX] Threading changes, implement OpenGL support, Font fix

[m4xw]

Description

Added some changes to the switch_pthread wrapper:

• Let the system schedule the threads across the first 3 cores

Implemented OpenGL support:

• Currently there is a bug in mesa's switch-EGL backend that doesn't allow clean re-init
• A workaround is to not terminate egl and just leak, but terminate it on platform de-init
• Added Symbols/Headers to fully load GL (since we can't use <GL/gl.h>)
• We need to check if we want to unify the glsym.py scripts, as they are all very simliar and make small changes for easier maintainability

Fix switch_font:

• Just a oversight from the initial backport

Notes

Currently HAVE_OPENGL is off by default, as it break's video_threaded which is a requirement to get fullspeed on some cores (mostly pcsx).
Both GL and non-GL build are functioning.
I made a quick test with mupen64plus and it just works if glsym files are replaced (ignoring all non-relevant changes).

Reviewers

@twinaphex @Themaister

[inactive123]

Looks good to me @m4xw. Just one question - is HAVE_XMB disabled deliberately when GL support is enabled? I would imagine that would be one of the nicer menu systems to use with GL support.

[m4xw]

@twinaphex HAVE_XMB is disabled right now because that needs threaded data runloop set to false.
Then it works. Material doesn't have that problem so I just focus on that for now.
No problem with ozone either btw.
Another note: Seems there is a fix for my deinit problem on upstream, tho not on the packages for libnx yet.
I can check that this evening myself, we can either wait with this PR or I just remove that workaround later

[inactive123]

I'm fine with merging this for now and then you just remove the workaround later. So let's do that.