new convention

The only call to the findDiscardLevelsBoundaries() method was commented out
inside initDecode(), with a comment:
        // Merov : Test!! DO NOT COMMIT!!

This was the only caller of copy_tile(), which was the only caller of
copy_block(). Commented out all three of these (biggish!) functions, since I
have no idea what any of them were supposed to do or when it might be useful
to call them. In other words, I can't yet rule out the possibility that I
might have to uncomment them.

Use boost::scoped_ptr instead of raw pointers to LLKDUMemSource,
LLKDUDecodeState, kdu_coords and kdu_dims so cleanup is simpler, and automated
on destruction of LLImageJ2CKDU.

Replace pointer to kdu_codestream with a custom RAII class. kdu_codestream is
itself an opaque handle, so we don't need to add another layer of indirection.
Just wrap it to ensure its destroy() method is reliably called when needed.

Make static instances of LLKDUMessageWarning and LLKDUMessageError
self-register, eliminating the companion static bool and explicit checks in
code.

instead of legacy BOOL.

Derive them both from a common base class that does the message logging,
instead of having each handler class log redundantly -- especially since the
put_text() override accepting const kdu_uint16* was simply streaming the
kdu_uint16 pointer to the log file, which would log the hex value of the
pointer.

Although we want a static instance of each of these handler classes, pull it
out rather than nesting the instance within the class itself.

Specifically, remove unused function pointer types CreateLLImageJ2CFunction,
DestroyLLImageJ2CFunction and EngineInfoLLImageJ2CFunction.

Also eliminate static fallbackDestroyLLImageJ2CImpl() and
fallbackEngineInfoLLImageJ2CImpl(), leaving only static
fallbackCreateLLImageJ2CImpl().

We do need a factory function to instantiate the appropriate LLImageJ2CImpl
subclass, so leave the fallbackCreateLLImageJ2CImpl() link seam in place.

However, given that every known LLImageJ2CImpl subclass is cheap to
instantiate, make getEngineInfo() a pure virtual method on that subclass: the
static LLImageJ2C::getEngineInfo() method can temporarily construct an
instance to query. While we're at it, make getEngineInfo() return std::string
like LLImageJ2C::getEngineInfo(). It's ridiculous that
fallbackEngineInfoLLImageJ2CImpl() implementations constructed a static
std::string and returned its c_str(), only to have LLImageJ2C::getEngineInfo()
construct ANOTHER std::string from the returned const char*.

fallbackDestroyLLImageJ2CImpl() never did anything useful: it merely deleted
the passed LLImageJ2CImpl subclass pointer as the specific subclass type. But
since LLImageJ2CImpl's destructor is virtual, LLImageJ2C's destructor could
simply delete the stored LLImageJ2CImpl*. In fact, make mImpl a
boost::scoped_ptr<LLImageJ2CImpl> so we don't even have to delete it manually.

destroyLLImageJ2CKDU().

These were apparently intended as simple C-style DLL entry points. But as
nobody calls them, and as we decided against building the viewer from DLLs,
they only clutter the code.

MAINT-6565: Grab a shared pointer and encapsulate it into the bind call in place of this. Ensures that the impl is not deleted while the coroutine is active.