Refactored pbr sample to use script rerun
Refactored pbr sample to use script rerun

Read "Use IBL" user var in main script execution rather than frame
callback. Use it to only compile the shader we need, rather than both.

Means you can iterate on the (fast-compiling) no-IBL version without
having to recompile the (very-slow-compiling) IBL version.

Also simplifies the script a bit.

simontaylor81/Syrup,simontaylor81/Syrup