[ivgl] Initialize variables before we use them.

[stolk]

Description

This addresses:

==126424== Conditional jump or move depends on uninitialised value(s)
==126424==    at 0x16A24E: IvGL::mouseMoveEvent(QMouseEvent*) (src/iv/ivgl.cpp:1208)
==126424==    by 0x5FCD5F6: QWidget::event(QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt6Widgets.so.6.4.2)
==126424==    by 0x5F843AF: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt6Widgets.so.6.4.2)
==126424==    by 0x5F7D2C6: QApplication::notify(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt6Widgets.so.6.4.2)
==126424==    by 0x6E35447: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt6Core.so.6.4.2)
==126424==    by 0x5F79D7D: QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) (in /usr/lib/x86_64-linux-gnu/libQt6Widgets.so.6.4.2)
==126424==    by 0x5FDBAD6: ??? (in /usr/lib/x86_64-linux-gnu/libQt6Widgets.so.6.4.2)
==126424==    by 0x5FDCE74: ??? (in /usr/lib/x86_64-linux-gnu/libQt6Widgets.so.6.4.2)
==126424==    by 0x5F843AF: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt6Widgets.so.6.4.2)
==126424==    by 0x6E35447: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt6Core.so.6.4.2)
==126424==    by 0x6762DE6: QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) (in /usr/lib/x86_64-linux-gnu/libQt6Gui.so.6.4.2)
==126424==    by 0x67AD5CB: QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQt6Gui.so.6.4.2)
==126424==  Uninitialised value was created by a heap allocation
==126424==    at 0x4846FA3: operator new(unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==126424==    by 0x11F509: ImageViewer::ImageViewer(bool, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (src/iv/imageviewer.cpp:140)
==126424==    by 0x17D248: main (src/iv/ivmain.cpp:127)

Tests

With this patch, this particular valgrind error goes away.

Tested by running valgrind on iv before and after change.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[lgritz]

LGTM