Add time function

Updates to objects should not be done in frame by frame basis, but by
time difference. This commit adds time diff function which can be used
to tell how much time has passed since last frame.

graphics/src/vulkan_commons.c

@@ -1,3 +1,26 @@
 #include "vulkan_commons.h"
 
-struct rse_vulkan_state_t g_vulkan_state;
+#include <time.h>
+
+struct rse_vulkan_state_t g_vulkan_state;
+
+time_t g_last_frame_time;
+time_t g_this_frame_time;
+float g_float_delta_time;
+
+void rse_init_time()
+{
+    time(&g_last_frame_time);
+}
+
+void rse_update_time()
+{
+    g_last_frame_time = g_this_frame_time;
+    time(&g_this_frame_time);
+    g_float_delta_time = (float)difftime(g_this_frame_time, g_last_frame_time);
+}
+
+float inline rse_get_time_diff()
+{
+    return g_float_delta_time;
+}