MaterialPipelineArgs has no way to describe how many SRV descriptors it needs

In #589 I found that MaterialPipelineArgs was creating more SRV descriptors than what the command buffer could bind. The need to specify SRV descriptors in the command buffer is a DirectX-ism.

I solved #589 by decreasing the number of SRV descriptors required. However, MaterialPipelineArgs should provide what it needs so that a command buffer can be created that satisfies its needs

Maybe another thing to consider: is there a command buffer implementation that doesn't require specifying the number of SRV descriptors needed? This is only required for DirectX