removed NumericType requirement for SourceAndConverter etc.
Paint a single VolatileShortType source (without hierarchy rendering, just the new type)
re-simplified Hdf5ImgCells generics
remove CacheIoTiming. no longer required if data is loaded asynchronously.
Cell loading in fetcher thread.
rendering using VolatileHierarchyProjector
remove unused classes
use a single VolatileShortArray for all empty cells
Replace fetcher BlockingDeque by (blocking) set of queues. The Queues have
different priorities, blocks with highest priority (lowest res) are loaded
first.

Queues are cleared for every new frame (working through screenscales
counts as a single frame) such that the blocks required for the current
frame are loaded immediately. The plan is to move the cleared-out requests
to a prefetch deque of limited size.

For queue-clearing, SpimViewer needs to know the Hdf5GlobalCache which is
not so nice. Maybe we can find a better solution later.
revise BlockingFetchQueue locking algorithm
reduce chatter
bug fix in VolatileHierarchyProjector:
Mask image always starts at zero, target might not.
repeatedly draw to the same screen-image without clearing the mask if possible.
only check screenscale when new projector was use. later passes write less
pixels and take less time, which may skew the results.
Render individual view to renderbuffer, blend renderbuffers for display.
screenImages of different resolution (but same doublebuffer) share data array
handle multiple visible sources
no need to create renderImages unless more than 1 source is visible
update wrt changes in imglib2
double-buffering requires 3 render buffers:
The Renderer alternates between two of those.
Whenever one is ready, it is proposed to the Painter, which will,
on the next paint() call, swap it for the currently rendered image
(this is the 3rd buffer).