MVKCmdWaitEvents: end current encoder before encodeWait

[dboyan]

The Metal documentation for encodeWaitForEvent method says:

A command buffer can instruct the GPU to wait for an event only between passes, not within a pass. If a command buffer has an active encoder, finish using the encoder, call its endEncoding() method, and then call this method before creating another encoder.

We should obey this rule and call endEncoding when there can be active encoder around.

This should fix #2078.

[cdavis5e]

My one concern is that splitting up Metal render passes like this would require special handling of e.g. DONT_CARE attachments whose intermediate results are used within a render pass--but we already have to deal with that because of tessellation, among other things. One of these days, someone should look into reducing the need to interrupt render passes like this...