Remove textures from irrlicht cache before game starts

[deveee]

Need to find out why it sometimes doesn't unload textures. Maybe there is something messed with texture grab/drop somewhere or maybe it's related to this, or maybe both.

==145625== 12,320 (12,032 direct, 288 indirect) bytes in 32 blocks are definitely lost in loss record 1,667 of 1,694
==145625==    at 0x4846FA3: operator new(unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==145625==    by 0x43D14C: GUIButtonImage::GUIButtonImage(irr::gui::IGUIEnvironment*, irr::gui::IGUIElement*, int, irr::core::rect<int>, ISimpleTextureSource*, bool) (src/gui/guiButtonImage.cpp:38)
==145625==    by 0x43D4E2: GUIButtonImage::addButton(irr::gui::IGUIEnvironment*, irr::core::rect<int> const&, ISimpleTextureSource*, irr::gui::IGUIElement*, int, wchar_t const*, wchar_t const*) (src/gui/guiButtonImage.cpp:83)
==145625==    by 0x46FC15: GUIFormSpecMenu::parseImageButton(GUIFormSpecMenu::parserData*, 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/gui/guiFormSpecMenu.cpp:2114)
==145625==    by 0x4787F1: GUIFormSpecMenu::parseElement(GUIFormSpecMenu::parserData*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (src/gui/guiFormSpecMenu.cpp:3048)
==145625==    by 0x47B665: GUIFormSpecMenu::regenerateGui(irr::core::vector2d<unsigned int>) (src/gui/guiFormSpecMenu.cpp:3505)
==145625==    by 0x47C570: GUIFormSpecMenu::drawMenu() (src/gui/guiFormSpecMenu.cpp:3704)
==145625==    by 0x4D7B65: GUIModalMenu::draw() (src/gui/modalMenu.cpp:99)
==145625==    by 0x433D14: irr::gui::IGUIElement::draw() (Irrlicht-deve/include/IGUIElement.h:337)
==145625==    by 0x84863C: irr::gui::CGUIStaticText::draw() (CGUIStaticText.cpp:142)
==145625==    by 0x433D14: irr::gui::IGUIElement::draw() (Irrlicht-deve/include/IGUIElement.h:337)
==145625==    by 0x81F8C9: irr::gui::CGUIEnvironment::drawAll(bool) (CGUIEnvironment.cpp:215)

[MoNTE48]

Please create separate PR from the second commit.

[deveee]

51babdd