Make Linux GetTicks() start from zero

Since the GetTicks() is visible in the logging, it's useful
to have it start from zero rather than an arbitrary number.

internal/c/libqb.cpp

@@ -29914,6 +29914,8 @@ int32 func__scaledheight() { return environment_2d__screen_scaled_height; }
 extern void set_dynamic_info();
 
 int main(int argc, char *argv[]) {
+    clock_init();
+
 
 #if defined(QB64_LINUX) && defined(X11)
     XInitThreads();

internal/c/libqb/include/datetime.h

@@ -4,6 +4,14 @@
 #include <stdint.h>
 #include "qbs.h"
 
+#ifdef QB64_LINUX
+// Initializes the clock returned by 'GetTicks()' so that it starts from zero
+// Should be called at the very beginning of the program
+void clock_init();
+#else
+static inline void clock_init() { }
+#endif
+
 int64_t GetTicks();
 
 double func_timer(double accuracy, int32_t passed);

internal/c/libqb/src/datetime.cpp

@@ -37,10 +37,18 @@ static int64_t orwl_gettime(void) {
 #endif
 
 #ifdef QB64_LINUX
+static int64_t initial_tick = 0;
+
+void clock_init() {
+    // When GetTicks() is called here initial_tick is zero, so as a result
+    // GetTicks() returns the original value of the clock.
+    initial_tick = GetTicks();
+}
+
 int64_t GetTicks() {
     struct timespec tp;
     clock_gettime(CLOCK_MONOTONIC, &tp);
-    return tp.tv_sec * 1000 + tp.tv_nsec / 1000000;
+    return (tp.tv_sec * 1000 + tp.tv_nsec / 1000000) - initial_tick;
 }
 #elif defined QB64_MACOSX
 int64_t GetTicks() { return orwl_gettime(); }