1. use `SIZE_T_ARG` instead of `"%lu"`; should fix format warning in #5780
2. since `HeapAllocator::checkRangesMerged()` has no effect unless running in debug, `#ifdef` out the function; should fix CI warning in #5778
This changes the model loading code such that all index and vertex data
is packed in a single GPU buffer for each data type (index and vertex
data are still in separate buffers). This reduces the amount of buffer
rebinding needed when rendering a scene which will reduce the CPU
overhead. Furthermore, this opens up possibilities of future
improvements for which it is beneficial if all data is in the same
buffer (e.g. MultiDrawIndirect rendering).
To manage where to put the data in the GPU buffer I implemented a
custom HeapAllocator which manages where data can be put. It uses a very
primitive first-fit algorithm but that is sufficient for this usage
type. This does add quite a lot of overhead while allocating GPU model
data (I did some tracing and some allocation calls took as long as
130ms).
This fixes#1457.
