ZVISION: Fix an off-by-one error in the RLF decoder

A regression from 7f61a09. The current frame is the currently
displayed frame, not the frame that should be displayed next. Thanks to
clone2727 and Marisa-Chan for the explanation and fixes

engines/zvision/video/rlf_decoder.cpp

@@ -56,7 +56,7 @@ RLFDecoder::RLFVideoTrack::RLFVideoTrack(Common::SeekableReadStream *stream)
 	  _height(0),
 	  _frameTime(0),
 	  _frames(0),
-	  _curFrame(-1),
+	  _displayedFrame(-1),
 	  _frameBufferByteSize(0) {
 
 	if (!readHeader()) {
@@ -161,11 +161,11 @@ bool RLFDecoder::RLFVideoTrack::seek(const Audio::Timestamp &time) {
 	uint frame = getFrameAtTime(time);
 	assert(frame < (int)_frameCount);
 
-	if ((uint)_curFrame == frame)
+	if ((uint)_displayedFrame == frame)
 		return true;
 
-	int closestFrame = _curFrame;
-	int distance = (int)frame - _curFrame;
+	int closestFrame = _displayedFrame;
+	int distance = (int)frame - closestFrame;
 
 	if (distance < 0) {
 		for (uint i = 0; i < _completeFrames.size(); ++i) {
@@ -189,18 +189,18 @@ bool RLFDecoder::RLFVideoTrack::seek(const Audio::Timestamp &time) {
 		applyFrameToCurrent(i);
 	}
 
-	_curFrame = frame;
+	_displayedFrame = frame - 1;
 
 	return true;
 }
 
 const Graphics::Surface *RLFDecoder::RLFVideoTrack::decodeNextFrame() {
-	if (_curFrame == (int)_frameCount)
+	if (_displayedFrame >= (int)_frameCount)
 		return NULL;
 
-	applyFrameToCurrent(_curFrame);
+	_displayedFrame++;
+	applyFrameToCurrent(_displayedFrame);
 
-	_curFrame++;
 	return &_currentFrameBuffer;
 }
 

engines/zvision/video/rlf_decoder.h

@@ -46,7 +46,7 @@ class RLFDecoder : public Video::VideoDecoder {
 		uint16 getWidth() const { return _width; }
 		uint16 getHeight() const { return _height; }
 		Graphics::PixelFormat getPixelFormat() const { return Graphics::PixelFormat(2, 5, 5, 5, 0, 10, 5, 0, 0); /* RGB 555 */ }
-		int getCurFrame() const { return _curFrame; }
+		int getCurFrame() const { return _displayedFrame; }
 		int getFrameCount() const { return _frameCount; }
 		const Graphics::Surface *decodeNextFrame();
 		bool isSeekable() const { return true; }
@@ -121,7 +121,7 @@ class RLFDecoder : public Video::VideoDecoder {
 		Frame *_frames;
 		Common::Array<uint> _completeFrames;
 
-		int _curFrame;
+		int _displayedFrame;
 		Graphics::Surface _currentFrameBuffer;
 		uint32 _frameBufferByteSize;