Fluid: Added offset to control frame range

Added an offset field to control when to load the simulation files. Since this is a very small but helpful addition it is in my view safe to commit at this point of the bcon cycle.

source/blender/makesdna/DNA_fluid_types.h

@@ -580,6 +580,7 @@ typedef struct FluidDomainSettings {
   int cache_frame_pause_mesh;
   int cache_frame_pause_particles;
   int cache_frame_pause_guide;
+  int cache_frame_offset;
   int cache_flag;
   char cache_mesh_format;
   char cache_data_format;
@@ -589,7 +590,7 @@ typedef struct FluidDomainSettings {
   char error[64]; /* Bake error description. */
   short cache_type;
   char cache_id[4]; /* Run-time only */
-  char _pad8[6];
+  char _pad8[2];
 
   /* Time options. */
   float dt;