Fix GridWrap crash when resized to same size before renderer created

[dweymouth]

Description:

GridWrap would crash if resized to its current size before its renderer is created (since BaseWidget.Resize to the same size is a no-op and doesn't create the renderer). I ran into this in Supersonic, in my system of widget reuse, and had worked around it but figured out the root cause and a proper fix.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 59.209% (+0.02%) from 59.189%
when pulling 5df62d2 on dweymouth:gridwrap-crash-before-renderer
into d7eeddc on fyne-io:develop.