add interface for keep Matrix
calculate viewport based on _fullRect _rtRect and _fullRectViewport
fix matrix push/pop bug
implement interface setKeepMatrix, setVirtualViewPort
add new test cases
set scaleY to -1 for test case, because renderTexture result is not the same as texture load from file
remove blank line，adjust indent
rename setVirtualViewPort->setVirtualViewport
add comment to make interface clearer
Merge pull request #5534 from dabingnn/develop_RenderTextureImprovement

Develop render texture improvement