[FIX] errors in image element when texture asset is not the registry

Merge branch 'master' of github.com:playcanvas/engine

Merge pull request playcanvas#804 from playcanvas/screenspace-materia…

…l-variant

Support for screenspace shaderdef

Merge pull request playcanvas#802 from playcanvas/camera-refactor

pc.Camera refactor

Merge pull request playcanvas#795 from guycalledfrank/fixioscube

Fix ios cubemaps after the mipmap change

[FIX] Revert script attribute change as it breaks attributes in Editor

Merge branch 'master' of github.com:playcanvas/engine

fix texture vram tracking

[FIX] only submit vrdisplay frame when actually presenting