rendy silently allows binding to, drawing from slots with no vertex format specified

To reproduce, edit the triangle example and remove the implementation of `TriangleRenderPipelineDesc::vertices` (so that the default implementation is used).

The example still compiles, runs and issues no warnings, but does not render the triangle. Because no vertex format has been specified, the shader fails to pull any data from the vertex buffer.

This should probably at least be checked in debug builds, and documented for this and any other descriptor method(s) that might be susceptible to the same problem.