MagnifyingGlass::sizeUp() and MagnifyingGlass::sizeDown() grow/shrink
both width and height, but check only width's limits. Thus the user can
first increase Magnifying glass's height, then increase its size and
make the height greater than the main window's height. The user can also
first increase the width, then decrease the size until the height
shrinks to 0 and Magnifying glass disappears.
When Magnifying glass disappears, the only way to make it visible again
is to restore its default size by restarting YACReader, because the
invisible MagnifyingGlass widget does not receive wheel events and
Viewer::keyPressEvent() propagates shortcuts to mglass only if it is
visible. And even this workaround is possible only because YACReader
does not save/restore Magnifying glass's size (should it?).
Always checking both width and height limits fixes the bug. If one of
the dimensions reaches a limit, only the other dimension is modified. If
both dimensions reach their limits, neither is modified.
Not all possible keyboard modifiers of a wheel event change
MagnifyingGlass's pixmap. So in case of e.g. MetaModifier or more than
one modifier, MagnifyingGlass::wheelEvent() calls updateImage() to no
avail.
Move the updateImage() call into the public slots to make them
self-sufficient. This also allows not to call updateImage() when the
pixmap is not changed in case the adjusted parameter has reached its
minimum or maximum value already.
