Fix MSVC warnings about "forcing value to bool"
Fix MSVC warnings about "forcing value to bool"

Prior to 3671e6a9bd19e03bb4a5d7c963c625cd6bd500cf, IS_SKIP was a
plain comparison with ==.

This fixes MSVC warnings like these:
codec/encoder/core/src/svc_base_layer_md.cpp(1864) : warning C4800: 'WelsEnc::Mb_Type' : forcing value to bool 'true' or 'false' (performance warning)

shihuade/openh264,GuangweiWang/openh264,shihuade/openh264,ruil2/openh264,GuangweiWang/openh264,mstorsjo/openh264,cisco/openh264,mstorsjo/openh264,shihuade/openh264,mstorsjo/openh264,ruil2/openh264,cisco/openh264,ruil2/openh264,shihuade/openh264,ruil2/openh264,GuangweiWang/openh264,mstorsjo/openh264,cisco/openh264,mstorsjo/openh264,GuangweiWang/openh264,cisco/openh264,cisco/openh264,GuangweiWang/openh264,cisco/openh264,mstorsjo/openh264,ruil2/openh264,ruil2/openh264,shihuade/openh264,shihuade/openh264,shihuade/openh264,cisco/openh264,ruil2/openh264,mstorsjo/openh264,GuangweiWang/openh264,GuangweiWang/openh264