Ruby is not guaranteed to delete bitmaps after any windows, sprites, etc that they are attached to. In the event that the attached object is not deleted before the next call to Graphics.update, this will most likely result in a segfault, as isDisposed() is not guaranteed to return true for a deleted bitmap.
Bitmap::invalid was added in an attempt to guard against this for sprites, but since the bitmap in question is deleted it's not guaranteed that the pointer actually points to null, which made the fix unreliable.
+ Time is now measured in fractions of a second, not microseconds
+ Viewports in RGSS1 will now only retain weak references to their children
+ Invalid Sprite bitmaps found during Graphics.update will be treated as null
+ Bitmap#mega? and Bitmap#animated? are properly rescued
