[platforms/hwcomposer] Few fixes for hwcomposer platforms

Summary:
- Actually use double layered compositing, previously both layers were
of type HWC_FRAMEBUFFER, instead of 2nd layer being
HWC_FRAMEBUFFER_TARGET. Thanks to @NotKit on telegram for pointing this
out.

- Use sourceCropf instead of sourceCrop, as sourceCrop is legacy option.

Test Plan: have to test on actual device, but test_hwcomposer does have similar code.

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8587