Convert rc_max_intra_bitrate_pct to control

Since this is the only ABI incompatible change since the last release,
convert it to use the control interface instead. The member of the
configuration struct is replaced with the VP8E_SET_MAX_INTRA_BITRATE_PCT
control.

More significant API changes were expected to be forthcoming when this
control was first introduced, and while they continue to be expected,
it's not worth breaking compatibility for only this change.

Change-Id: I799d8dbe24c8bc9c241e0b7743b2b64f81327d59