SurfaceItem: Use `pixmap()` for quad generation

This uses the previous pixmap if the current one is invalid, which ensures the generated quads have sane coordinates.

The issue caused flickering while resizing Chrome after 2f4fa23e

While at it, move size out of the loop as it doesn't change, and add a null check before generating the quads as we now access the pixmap unconditionally outside the region loop.

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Kai Uwe Broulik kai_uwe.broulik@mbition.io on behalf of MBition GmbH

https://github.com/mercedes-benz/foss/blob/master/PROVIDER_INFORMATION.md