misc: Switch from gl3w to glad

Also add more checks on window and context creation.

src/main.cpp

@@ -1,11 +1,7 @@
 #include "emulator.hpp"
-#include "gl3w.h"
 
 int main (int argc, char *argv[]) {
     Emulator emu;
-    if (gl3wInit()) {
-        Helpers::panic("Failed to initialize OpenGL");
-    }
 
     emu.initGraphicsContext();