Add validation for equirect and cylinder layers' init values

[svillar]

Layers like equirect or cylinder have specific values in their init structures that define how they look like. The specs do define some constraints for those values inline but do not define specific steps for validating those values.

I guess that means that UAs should clamp those values as there is no specified error to be returned. Clamping on the one hand allows sessions to run even with invalid values but on the other side generate non expected results. What would be the recommendation?

[cabanier]

Sorry it took me so long to address this. See #341 which should fix this.