* gtk3: Add base for future Vulkan integration
This PR puts in place the fondation for the future Vulkan integration on
the GTK3 UI.
This also updated SPB to 0.0.3-build14 that fixed a use after free on
XErrorHandler on Linux.
* Address rip's comments
* Merge GLWidget inside GLRenderer
* Clean up and deduplicate renderer implementations
* Address shahil's comments
* Address Ac_K's comments
* Address gdkchan's comments
