ImageButton texture is not scaled properly

If ImageButton texture height is different from the widget and the ratio is not an integer it will not be properly scaled with int scale factor.

For example textures/tx_menubook_quests_all_idle.dds from https://www.nexusmods.com/morrowind/mods/46854 (HD texture buttons (English)) scaled to height of 127 pixels is rendered as following without this MR because int scale = 127 / 32 is 3 when float is 3.96875.