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-video-gl3.c

@@ -559,7 +559,7 @@ static glsl_t* load_glslp(glsl_t* glsl, int num_shader, const char* f)
         if (p)
         {
                 char path[512];
-                char file[512];
+                char file[1024];
                 int failed = 0;
                 strcpy(path, f);
                 char* filename = get_filename(path);