Don't remove background when resizing

This leads to flickers, as we should not draw in a translucent way
if we didn't specify that our window is translucent, because the
renderer has some about translucency.

Instead, we should clip the backdrop content by the inverse of the
content clip rect, which is not yet implemented.

Bug: 28009524
Change-Id: Ia3f54fb83997ace863e78ff1cbe45cfb64f92f26