Fix slowdown of _ScreenX and _ScreenY

The commands _ScreenX and _ScreenY got significantly slower due to the
need to wait for the GLUT thread to wake up and execute the glutGet()
command for them. We've already seen a few programs (including the IDE)
where this behavior completely grinds the program to a halt, so we
definitely can't keep it.

The simple solution here is to not call glutGet() on every _ScreenX/Y
command. Instead every time the idle/timer function runs we get the
current values for the relevant glutGet() variables and store them.
libqb_glut_get() then checks if the value being read is one of the ones
we read in the idle/timer functionand if so just returns the last read
value. By doing it this way the commands no longer has to wait on the
GLUT thread for the result.

internal/c/libqb/src/glut-msg-queue.cpp

@@ -4,13 +4,30 @@
 #include <unistd.h>
 #include <queue>
 
+// note: MacOSX uses Apple's GLUT not FreeGLUT
+#ifdef QB64_MACOSX
+#    include <GLUT/glut.h>
+#else
+#    define CORE_FREEGLUT
+#    include "freeglut.h"
+#endif
+
 #include "mutex.h"
 #include "glut-message.h"
 #include "glut-thread.h"
 
 static libqb_mutex *glut_msg_queue_lock = libqb_mutex_new();
 static std::queue<glut_message *> glut_msg_queue;
 
+// These values from GLUT are read on every process of the msg queue. Calls to
+// libqb_glut_get() can then read from these values directly rather than wait
+// for the GLUT thread to process the command.
+static int glut_window_x, glut_window_y;
+
+#ifdef CORE_FREEGLUT
+static int glut_window_border_width, glut_window_header_height;
+#endif
+
 bool libqb_queue_glut_message(glut_message *msg) {
     if (!libqb_is_glut_up()) {
         msg->finish();
@@ -27,6 +44,14 @@ bool libqb_queue_glut_message(glut_message *msg) {
 void libqb_process_glut_queue() {
     libqb_mutex_guard guard(glut_msg_queue_lock);
 
+    glut_window_x = glutGet(GLUT_WINDOW_X);
+    glut_window_y = glutGet(GLUT_WINDOW_Y);
+
+#ifdef CORE_FREEGLUT
+    glut_window_border_width = glutGet(GLUT_WINDOW_BORDER_WIDTH);
+    glut_window_header_height = glutGet(GLUT_WINDOW_HEADER_HEIGHT);
+#endif
+
     while (!glut_msg_queue.empty()) {
         glut_message *msg = glut_msg_queue.front();
         glut_msg_queue.pop();
@@ -45,7 +70,34 @@ void libqb_glut_warp_pointer(int x, int y) {
     libqb_queue_glut_message(new glut_message_warp_pointer(x, y));
 }
 
+static bool is_static_glut_value(int id) {
+    return id == GLUT_WINDOW_Y
+        || id == GLUT_WINDOW_X
+#ifdef CORE_FREEGLUT
+        || id == GLUT_WINDOW_BORDER_WIDTH
+        || id == GLUT_WINDOW_HEADER_HEIGHT
+#endif
+    ;
+}
+
+static int __get_static_glut_value(int id) {
+    switch (id) {
+    case GLUT_WINDOW_Y:             return glut_window_y;
+    case GLUT_WINDOW_X:             return glut_window_x;
+#ifdef CORE_FREEGLUT
+    case GLUT_WINDOW_BORDER_WIDTH:  return glut_window_border_width;
+    case GLUT_WINDOW_HEADER_HEIGHT: return glut_window_header_height;
+#endif
+    default:                        return -1;
+    }
+}
+
 int libqb_glut_get(int id) {
+    if (is_static_glut_value(id)) {
+        libqb_mutex_guard guard(glut_msg_queue_lock);
+        return __get_static_glut_value(id);
+    }
+
     glut_message_get msg(id);
 
     libqb_queue_glut_message(&msg);