Fix compiler warning about signed/unsigned mismatch in CPrimitiveMaterialBatcher

[patrikjuvonen]

CPrimitiveMaterialBatcher::Flush has a for-loop which is using an int instead of an uint when iterating using m_primitiveList.size(), this causes a compiler warning about signed/unsigned mismatch, as the size is an unsigned value. Tiiiiiny quality of life change, but let's avoid warnings in compiler.

[qaisjp]

Thanks!