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.c

@@ -163,9 +163,9 @@ void get_glslp_name(const char* f, char* s, int size)
 glslp_t* glslp_parse(const char* f)
 {
         int i, j, len, sublen;
-        char s[512], t[512], z[512];
+        char s[512], t[512], z[540];
 
-        s[511] = 0;
+        memset(s, 0, sizeof(s));
         if (endswith(f, ".glsl"))
                 return glsl_parse(f);