Support assigning a python dict in shader cursor api

[ccummingsNV]

Compute kernel dispatch currently supports a very convenient means to set variables by providing a dictionary, however this isn't visible (or obvious) when using a shader cursor. This means that if you want to dispatch a compute pipeline as part of a series of commands in a command buffer, you have to use a more complex api to pass uniforms. We should expand the dispatch helper to function with shader cursors in general.