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-glslp-parser.h

@@ -22,7 +22,7 @@ struct texture {
 };
 
 struct shader {
-        char shader_fn[512];
+        char shader_fn[1024];
         char* shader_program;
         char alias[64];
         int filter_linear;