add version of gl debug callback for not-windows

also add note for differing signature, look into why this "works" on
windows and doesnt on linux later.. seems potentially bad

source/app.d

@@ -214,7 +214,9 @@ struct Chip8Status {
 	Assembly ass_;
 
 	// loaded program
+	const(char)*[string] string_cache;
 	const (char)* loaded_program;
+	string loaded_program_str;
 
 	// mem editor
 	// MemoryEditor mem_editor_;
@@ -265,8 +267,7 @@ struct Chip8Status {
 
 		import std.file : read;
 
-		loaded_program = "chip8_picture.ch8";
-		auto buf = read("programs/chip8_picture.ch8");
+		auto buf = read(loaded_program_str);
 		emu_.load(0x200, buf); // do ze load yes, will copy all the data in
 		ass_.setRange(cast(ushort)0x200, cast(ushort)(0x200 + cast(ushort)buf.length));
 		ass_.reset();
@@ -311,9 +312,22 @@ struct Chip8Status {
 					resetShortcut();
 				}
 
-				if (igMenuItem("Load", "CTRL+L")) {
+				if (igBeginMenu("Load")) {
+					import std.file;
+					auto dir_iterator = dirEntries("", "*.ch8", SpanMode.breadth);
+					foreach (DirEntry e; dir_iterator) {
+						import std.string : toStringz;
+						const (char)* c_str;
+						if (!(e.name in string_cache)) c_str = toStringz(e.name);
+						else c_str = string_cache[e.name];
+						if (igMenuItem(c_str)) {
+							loaded_program_str = e.name;
+							loaded_program = c_str;
 							loadShortcut();
 						}
+					}
+					igEndMenu();
+				}
 
 				if (igMenuItem("Debug", "CTRL+D")) {
 					debugShortcut();

source/window.d

@@ -29,6 +29,7 @@ struct Window {
 	SDL_Window* window;
 	SDL_GLContext context;
 
+	version(Windows) {
 		extern(Windows) nothrow @nogc
 		static void openGLCallbackFunction(
 			GLenum source, GLenum type,
@@ -47,6 +48,26 @@ struct Window {
 			}
 
 		} // openGLCallbackFunction
+	} else {
+		extern(C) nothrow @nogc
+		static void openGLCallbackFunction(
+			GLenum source, GLenum type,
+			GLuint id, GLenum severity,
+			GLsizei length, const (GLchar*) message, // TODO: look at why it's GLchar* for non-windows and the other works on windows..
+			void* userParam)
+		{
+
+			printf("Message: %s \nSource: %s \nType: %s \nID: %d \nSeverity: %s\n\n",
+				message, to!(char*)(source), to!(char*)(type), id, to!(char*)(severity));
+
+			if (severity == GL_DEBUG_SEVERITY_HIGH) {
+				printf("Aborting...\n");
+				import core.stdc.stdlib : exit;
+				exit(-1);
+			}
+
+		} // openGLCallbackFunction
+	}
 
 	void createWindow(int w, int h, const char* title = "Chipd8 Emu") {
 		assert(w > 0, "window width must be > 0");