Partial cleanup of timing system, with some guidance from Joshua:

* Fractional frames support has been changed to use a new var, scene->r.subframe. 
This is a 0.0-1.0 float representing a subframe interval, used in generating a final float 
frame number to evaluate animation system etc.
* Changed frame_to_float() and some instances of bsystem_time() into a convenience function:
float BKE_curframe(scene) which retrieves the floating point current frame, after subframe
and frame length corrections.
* Removed blur_offs and field_offs globals. These are now stored in render, used to 
generate a scene->r.subframe before render database processing.

source/blender/modifiers/intern/MOD_build.c

@@ -41,6 +41,7 @@
 #include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_particle.h"
+#include "BKE_scene.h"
 
 
 static void initData(ModifierData *md)
@@ -106,7 +107,7 @@ static DerivedMesh *applyModifier(ModifierData *md, Object *ob,
 		frac = bsystem_time(md->scene, ob, md->scene->r.cfra,
 					bmd->start - 1.0f) / bmd->length;
 	} else {
-		frac = md->scene->r.cfra - bmd->start / bmd->length;
+		frac = BKE_curframe(md->scene) - bmd->start / bmd->length;
 	}
 	CLAMP(frac, 0.0, 1.0);
 

source/blender/modifiers/intern/MOD_collision.c

@@ -40,6 +40,7 @@
 #include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_pointcache.h"
+#include "BKE_scene.h"
 
 
 static void initData(ModifierData *md) 
@@ -119,7 +120,7 @@ static void deformVerts(
 		CDDM_apply_vert_coords(dm, vertexCos);
 		CDDM_calc_normals(dm);
 		
-		current_time = bsystem_time (md->scene,  ob, ( float ) md->scene->r.cfra, 0.0 );
+		current_time = BKE_curframe(md->scene);
 		
 		if(G.rt > 0)
 			printf("current_time %f, collmd->time %f\n", current_time, collmd->time);

source/blender/modifiers/intern/MOD_explode.c

@@ -39,13 +39,14 @@
 #include "BLI_edgehash.h"
 
 #include "BKE_cdderivedmesh.h"
+#include "BKE_deform.h"
 #include "BKE_lattice.h"
 #include "BKE_mesh.h"
 #include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_particle.h"
+#include "BKE_scene.h"
 #include "BKE_utildefines.h"
-#include "BKE_deform.h"
 
 #include "MEM_guardedalloc.h"
 
@@ -682,7 +683,7 @@ static DerivedMesh * explodeMesh(ExplodeModifierData *emd,
 	timestep= psys_get_timestep(&sim);
 
 	//if(part->flag & PART_GLOB_TIME)
-		cfra=bsystem_time(scene, 0,(float)scene->r.cfra,0.0);
+		cfra= BKE_curframe(scene);
 	//else
 	//	cfra=bsystem_time(scene, ob,(float)scene->r.cfra,0.0);
 

source/blender/modifiers/intern/MOD_wave.c

@@ -38,6 +38,7 @@
 #include "BKE_DerivedMesh.h"
 #include "BKE_object.h"
 #include "BKE_deform.h"
+#include "BKE_scene.h"
 
 #include "depsgraph_private.h"
 
@@ -247,7 +248,7 @@ static void waveModifier_do(WaveModifierData *md,
 	MVert *mvert = NULL;
 	MDeformVert *dvert = NULL;
 	int defgrp_index;
-	float ctime = bsystem_time(scene, ob, (float)scene->r.cfra, 0.0);
+	float ctime = BKE_curframe(scene);
 	float minfac =
 			(float)(1.0 / exp(wmd->width * wmd->narrow * wmd->width * wmd->narrow));
 	float lifefac = wmd->height;