- UniformBuffer::toBufferDescriptor() now cleans the dirty flags
- SamplerBuffer move ctor and operator now clean the dirty flags of the
moved (source) buffer.
- SamplerBuffer::toCommandStream() can be used to more efficiently copy
a SamplerBuffer into the command stream -- this saves a copy.
- simplified SamplerBuffer's copy and move ctor/operator
- added a few constexpr here and there