Fix T42641, Graphical fragments showing on Blender 2.71 and higher when baking.

Safe for 2.73...

This revert rB9b0ab890676790bb1e8e77797629b889ea66f69e - needed to set that threshold to a small
negative value to remove the last artefacts reported in T39735, but now I could not reproduce
any with the previous 0.0f value, so restoring it for the time being.

If this 'shadowed neighbor face' case re-appears, we can always choose a value in-between, like -1e-18f...

source/blender/render/intern/include/rayintersection.h

@@ -121,7 +121,7 @@ typedef struct Isect {
 
 /* arbitrary, but can't use e.g. FLT_MAX because of precision issues */
 #define RE_RAYTRACE_MAXDIST	1e15f
-#define RE_RAYTRACE_EPSILON -FLT_EPSILON
+#define RE_RAYTRACE_EPSILON 0.0f
 
 #ifdef __cplusplus
 }