Validate ScinthDefs against maxBoundDescriptorSets

[lnihlen]

With envelopes and images both using samplers, there's a possibility users could author ScinthDefs that exceed the maximum supported number of simultaneously bound descriptor sets. Some statistics on gpuinfo show that a significant percentage of current hardware support a maximum of only 8.

The server should read from the VkPhysicalDeviceLimits structure on device init to get the limit, and fail to accept ScinthDefs with a descriptive error message when the limit is exceeded.