Fix buffer overflow warnings in OpenGL 3.0 video display file handling

Patch from davefiddes. Original commit message :

This fixes issues reported by gcc 10 with the size of various filename buffers
when concatenating strings into them.

Tests:
 - Build cleanly with gcc 10
 - Run a Pentium MMX Win98 machine and use custom GLSLP shader

src/wx-sdl2.c

@@ -504,7 +504,7 @@ int pc_main(int argc, char** argv)
         paths_init();
 
 #ifdef __linux__
-        char s[512];
+        char s[1024];
         /* create directories if they don't exist */
         if (!wx_setup(pcem_path))
                 return FALSE;