Add validation for structs decorated as Block or BufferBlock.

Fixes #937

Stop std140/430 validation when runtime array is encountered.

Check for standard uniform/storage buffer layout instead of std140/430.

Added validator command line switch to skip block layout checking.

Validate structs decorated as Block/BufferBlock only when they
are used as variable with storage class of uniform or push
constant.

Expose --relax-block-layout to command line.

dneto0 modification:
- Use integer arithmetic instead of floor.