Refactor initializing defaults in a SEncParamExt to a separate function
Add a new public method for initializing SEncParamExt to default values

This simplifies forward source compatibility when new fields are
added to SEncParamExt - when new fields are added to SEncParamExt,
this method makes sure those fields are initialized to the
default value - otherwise all API users would have to manually check
SEncParamExt every time it is updated to make sure there's no new fields
that should be set to a nonzero value by default (e.g. like
bEnableFrameSkip).
Renumber vtable entries in the C ABI test
Merge pull request #362 from mstorsjo/encoder-default-params

Add a new public method for initializing SEncParamExt to default values