Fix scrolling in scroll containers

[appgurueu]

Should fix #14517; since this is a highly platform and compiler-specific issue, I am hesitant to claim that it always works, so make sure to test this thoroughly.

It seems ButtonStates was not initialized for scroll events, but accessed through isLeftPressed. With optimizations enabled this probably yielded nonzero values, which resulted in a mess: The scroll container sometimes falsely believed that the button was held down.

How to test

Compile a release build with GCC on Linux. Verify that scrolling is choppy. Verify that with this PR applied, it isn't anymore. The following diff might help for testing:

diff --git a/irr/src/CIrrDeviceSDL.cpp b/irr/src/CIrrDeviceSDL.cpp
index 1a7fe0c5a..6437e8107 100644
--- a/irr/src/CIrrDeviceSDL.cpp
+++ b/irr/src/CIrrDeviceSDL.cpp
@@ -19,6 +19,7 @@
 #include <cstdlib>
 #include "SIrrCreationParameters.h"
 #include <SDL_video.h>
+#include <iostream>
 
 #ifdef _IRR_EMSCRIPTEN_PLATFORM_
 #include <emscripten.h>
@@ -666,6 +667,8 @@ bool CIrrDeviceSDL::run()
                case SDL_MOUSEWHEEL: {
                        SDL_Keymod keymod = SDL_GetModState();
 
+                       std::cout << "sdl says we scrollin" << std::endl;
+
                        irrevent.EventType = irr::EET_MOUSE_INPUT_EVENT;
                        irrevent.MouseInput.Event = irr::EMIE_MOUSE_WHEEL;
 #if SDL_VERSION_ATLEAST(2, 0, 18)
diff --git a/src/gui/guiScrollContainer.cpp b/src/gui/guiScrollContainer.cpp
index 2d71f3453..7015ed9c3 100644
--- a/src/gui/guiScrollContainer.cpp
+++ b/src/gui/guiScrollContainer.cpp
@@ -18,6 +18,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 */

 #include "guiScrollContainer.h"
+#include <iostream>
 
 GUIScrollContainer::GUIScrollContainer(gui::IGUIEnvironment *env,
                gui::IGUIElement *parent, s32 id, const core::rect<s32> &rectangle,
@@ -41,6 +42,8 @@ bool GUIScrollContainer::OnEvent(const SEvent &event)
                Environment->setFocus(m_scrollbar);
                bool retval = m_scrollbar->OnEvent(event);
 
+               std::cout << "scrollin" << std::endl;
+
                // a hacky fix for updating the hovering and co.
                IGUIElement *hovered_elem = getElementFromPoint(core::position2d<s32>(
                                event.MouseInput.X, event.MouseInput.Y));

Without the fix, not all SDL events "come through", even though you're not pressing any mouse buttons: You see "SDL says we scrollin" without "scrollin". After the fix, each "SDL says we scrollin" triggered inside the scroll container, without the mouse button being pressed, should be followed by a "scrollin".

[grorp]

You ninja'd me very hard, I just wanted to create a PR with the same patch (I was working on the same code to implement highdpi support, then noticed the uninitialized field). So yes, I can confirm that this works for me.

[Desour]

Tested, works. 👍 Thanks! <3

(But fix the comment, see sfan5's comment.)

[appgurueu]

Bump, is it good to go now?

(Asking since I ended up "fixing the comment" by always initializing the variable as appropriate, and I want to make sure I didn't miss something there.)