Remove ResetClip from Drawing API

The previous Mac backend implementation was incomplete and added
overhead to drawing operations-- CoreGraphics doesn't have any
facility for expanding the clipping region except via Save/Restore.
Since all backends include the clipping region in the save state,
let's just stick to a Save/Restore for this functionality.
[Mac] Fix ContextBackendHandler.DrawImage implementation

It was drawing the image upside down.
Fixes that poor cow in the Partial Images sample.
[Mac] Fix ContextBackendHandler implementations of TransformPoint(s) and TransformDistance(s)

The issue was that the matrix returned by CGContextGetCTM includes the full view transform,
but we only care about the view-relative transform applied to the context. Fix that by
saving the initial CTM of the context and concatenating its inverse whenever we need to get the
context transform. This should be safe as the initial affine transform returned by CoreGraphics
for an NSView should never be singular. I think.
[Xwt.Wpf] Use TextWrapping instead of TextTrimming for Label
Revert "Remove ResetClip from Drawing API"

This reverts commit 552b2354c992ca241fdea92f6fd4593d38bce20f.

Conflicts:
	Xwt.Mac/Xwt.Mac/ContextBackendHandler.cs
[Xwt.Gtk] Use GtkEngine.ReplaceChild when setting multiple time .Content on a custom widget
Revert "Revert "Remove ResetClip from Drawing API""

This reverts commit b0acfe1cf56ff31fe926546258c31e1a950970d0.
[Xwt] Place dialogs correctly

Automatic placement on mac works correctly now so remove the hack to
replace the window. This should fix placement on both Windows and
Mac so the dialogs are correctly centred.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=9012
[Xwt.Wpf] Position dialogs centred on owner by default

Fixes the Wpf side of bug 9012
[Xwt] If the native window is null, return null

This is a more logical behaviour and prevents us from hitting silly
null reference exceptions later on in the apps lifespan.

This probably explains some mysterious null reference exceptions we've
been seeing for the last few weeks.
[Mac] Don't depend on view.Layer for BackgroundColor since we're not using CoreAnimation

NSView.Layer is only set if using CoreAnimation and WantsLayer is set to True.
Instead, Canvas will manually fill its background color before calling OnDraw.
Other widgets that need to respect BackgroundColor will likely need to do something similar.