This should make testing easier, as Synth can be assumed to stay the same
during each call. Synth is also the part that we can parse from .asm/.json file
and a Patch can be compiled into a synth. Synth can be eventually made
quite opaque to the user. The user should not need to worry about opcodes
etc.
test_render_samples_api.c was added to test the api. bridge.go was modified to reflect that there is no need to check for row manually; su_render_samples already returns the information if a row has ended.
The main interface is render_samples function, which renders several samples in one call,
to limit the number of calls from Go to C. This is compiled into a library, which is then
linked and called from bridge.go.
