added dynamic amplitude, seed, type to noise generation

This commit also removes some debugging code accidentally left in from
the previous commit.

This also fixes a bug in the WASAPI backend. Thanks to Digital Mars for
picking that one up!

Added API to change waveform type dynamically

Support compiling with OpenWatcom

This changes a few things for custom backends and the calculation of
the size of the buffer. See the custom_backend example.

Public issue https://github.com/mackron/miniaudio/issues/222

This commit also fixes some bugs in the WASAPI backend.

This includes changes to callbacks used by custom backends. It adds a
`pConfig` parameter to both onContextInit and onDeviceInit. This allows
custom backends to specify custom config properties.

With the introduction of custom backends, it became necessary to
simplify the backend callbacks. For backwards compatibility, all
built-in backends are still using the old callbacks. Future commits are
going to be transitioning built-in backends to the new system. This
commit is designed to help with this migration so that we can migrate
backends one at a time while still keeping compatibility with this old
system, thereby allowing backends that haven't yet transitioned to keep
working.