pdInput: pass a (ByRef) override to all key functions

In most cases, a deliberately hooked key event will want to "eat" the
key event after handling, so other places in PD don't erroneously
duplicate the keypress.

In rare cases, however, that may not be desired; for example, I'm
working on allowing the user to use Tab to move through individual
entries in the layer box, and when they reach the end of the layer list,
another tab press will simply tab to the next control in line.  This
turns each layer entry in the box into its own "control", so to speak.

If an event receiver wants the default key handler to receive a hooked
event, it can now use the ByRef override to signal the master hook
function to pass the keypress on to its default handler.
pdCanvas: render the image to a dedicated picture box (rather than to the UC itself)

I'm loathe to make this change, but I think it's the only way to solve
issues with keyboard focus, as VB refuses to allow a UC with child
controls to have focus.  (Focus will always be forcibly passed to a
child.)

Rather than muck about with complicated subclassing to solve the
problem, I'll just use the VB-specific solution.  Now, we now longer
need to manually invoke SetFocus to prevent arrow keypresses from going
to unwanted controls.
pdWindowManager: new function for activating system-level double-buffering of hWnds

Still experimental, but may be used as part of other window
optimizations
pdCanvas: force refresh of some problematic buttons

I am extremely eager to replace jcButton with my own custom button, but
that has large repercussions across PD, so I think I may leave it until
post-6.4, just to be safe.

In the meantime, this fixes the Image Size button sometimes not drawing
after first load.
Improve flicker by forcing control clipping on some containers

I forgot that picture boxes don't automatically inherit the clipping
behavior of their parent form - argh.
Minor code clean-up to prep for 6.4 beta

The usual changes: better variable naming, removing large blocks of
commented-out test code, improving comments