When `Keep Quads` option is enabled in Import Settings of a mesh, Progressive Lightmapper gets stuck.

Steps for reproduction:
1) Open the scene `t1`
2) Ensure that backend is set to Progressive Lightmapper
3) Build the lighting
4) Observe the bug, baking gets stuck
5) Following error is thrown in the console log:
'PVR GBuffer' job failed with error code: 2 ('Non-triangle topology is not supported when creating the GBuffer.').

Notes:
- Not a regression, introduced when Progressive Lightmapper is introduced in 5.6.0b6
- Comparison is provided in the screenshot below, baking gets successfully completed when `Keep Quads` option is disabled