Move uniforms to command buffer scope

[thatcosmonaut]

Addresses #54

Uniform data is now scoped to the command buffer. I also addressed some D3D11 implementation inconsistencies that should improve performance via the Don't Touch The Driver principle.

Needs corresponding Metal changes.

[thatcosmonaut]

• Went back to a pooling strategy for uniform buffers to address situations where lots of command buffers are created and submitted (for example, when there is backpressure in immediate/mailbox mode).
• The number of uniform slots per stage is now 4 instead of 14.
• Uniform buffers are now 32 KiB instead of 1 MiB.