Closes #579, and some general improvements, ref #19.

This fixes that 64-bit data could be passed to a buffer without pygfx complaining about it, resulting in a broken visualization.

• Data passed to Geometry(...) is simply passed to the buffer constructor, so any checks happen in one place.
• The format is now derived in the Buffer and Texture constructor, instead of lazily. Fail early.
• Fix the detection of float64 arrays and also detect int64 arrays (The original issue).
• Buffer.format is now None for e.g. uniform buffers (instead of raising an error).
• Added some unit tests for buffer, texture and geometry.