Merge pull request #2348 from dingpinglv/Iss5968_RefactorCanvasRenderer

Issue #5968: refactor load event of texture, sprite frame and sprite etc
remove the useless _handleScissor which broke scissor clip
Merge remote-tracking branch 'cocos/Renderer' into new_renderer
Merge pull request #2349 from joshuastray/new_renderer

remove the useless _handleScissor which broke scissor clip
Fixed a bug of cc.view that it throws an error when html page doesn't add viewport meta