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);
 

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;

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);

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;