[GTK] Fix size request issue for canvas inside a ScrollView
[GTK] Fix size request issue for canvas inside a ScrollView

When doing size calculation in two steps (because of height for
width), in the first step don't reset the height, just return the
default value.
If the height is reset and the widget is inside a ScrollView, the
ScrollView will reset the scroll position.
steffenWi/xwt,lytico/xwt,antmicro/xwt,cra0zy/xwt,mminns/xwt,mono/xwt,hamekoz/xwt,iainx/xwt,directhex/xwt,sevoku/xwt,hwthomas/xwt,mminns/xwt,residuum/xwt,akrisiun/xwt,TheBrainTech/xwt