Add support for loading GL symbols using dlsym() instead of libretro API

[hhromic]

• Required for platforms with EGL version < 1.5
• To enable, use new compile-time define: -DGL_USE_DLSYM

Currently only enabled for Raspberry Pi platforms that use the legacy Broadcom driver.

[hhromic]

Btw, is there a function to cleanly exit on error from gliden? Right now if the lib is not found it logs the error but the plugin will still crash afterwards. I assume exit() is no bueno here?

[hhromic]

New version of the PR now. I found out that dlsym() can be called with RTLD_DEFAULT as the handle parameter and will search symbols in all the linked libraries, including the GL lib. We don't need to supply a hard-coded path nor use dlopen()/dlclose() anymore thus greatly simplifying the fix.

RTLD_DEFAULT
Find the first occurrence of the desired symbol using the default shared object search order. The search will include global symbols in the executable and its dependencies, as well as symbols in shared objects that were dynamically loaded with the RTLD_GLOBAL flag.

[m4xw]

@hhromic Can you please target the 2.0.5-RC1 branch, then I can merge it (will need some changes for thr branch anyway)
Also please amend and remove the @ next to my name.

[hhromic]

@m4xw rebased and commit msg amended. Test pending.
I'm going to test this change and the v2.0.5-RC1 later tonight on my RPI3B with legacy drivers to confirm all is fine to merge. Thanks!

Ps: I never liked the YOLO_DLSYM name for the define, I think this core deserves seriousness in its variable names :) Therefore I renamed it to GL_USE_DLSYM. I hope you don't mind.

[m4xw]

Whatever works, just give me the go when you are ready.

[hhromic]

Finished testing it now. All is working on RPI3B + Legacy Drivers 👍

I tested the rebased version (v2.0.5-RC1 branch) and the core is working pretty much as usual for this setup.

[m4xw]

Ty