[flash.filters] Add stubs for the missing filters

ConvolutionFilter, DisplacementMapFilter, GradientBevelFilter,
ShaderFilter
[DisplayObject] Fix a segfault when the object is not on the stage
[Loader] Keep track of aborted jobs until they have terminated

Before this fix, when load() and unload() were called several times in
a row, we sometimes discarded the pointer to the active thread and
kept a dangling pointer to a deleted thread instead.