fix(iv): Initialize variables before we use them. (#4457)

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)
```


With this patch, this particular valgrind error goes away.

Tested by running valgrind on iv before and after change.


Signed-off-by: Bram Stolk <b.stolk@gmail.com>

Author: stolk

src/iv/ivgl.cpp

@@ -54,6 +54,9 @@ IvGL::IvGL(QWidget* parent, ImageViewer& viewer)
     , m_centerx(0)
     , m_centery(0)
     , m_dragging(false)
+    , m_mousex(0)
+    , m_mousey(0)
+    , m_drag_button(Qt::NoButton)
     , m_use_shaders(false)
     , m_use_halffloat(false)
     , m_use_float(false)