[2opremio]

This adds WithAdvancedControl() to NewRenderContextSW and NewRenderContextGL, and WithBlockForTargetTime(bool) to RenderSW and RenderGL, in both builds. The options are variadic, so existing calls compile unchanged, and the defaults match libmpv's. WithAdvancedControl documents the promise render.h asks for: set the update callback right after creation and never wait for the core from the render thread.

A new test renders through the software backend with both options on. I vetted and compiled the purego build, but couldn't run its tests on macOS: MacPorts' libmpv fails to dlopen there, on unmodified main too.