fix using quad to clear framebuffer, but not correctly set scissor rect bug (#1913)
fix using quad to clear framebuffer, but not correctly set scissor rect bug (#1913)

* fix devide by zero in texture viewer when construct orientation matrix

* fix using quad to clear framebuffer, but not set scissor rect, cause clear failed bug

emoon/bgfx,jdryg/bgfx,jpcy/bgfx,jpcy/bgfx,emoon/bgfx,LWJGL-CI/bgfx,LWJGL-CI/bgfx,jdryg/bgfx,LWJGL-CI/bgfx,jpcy/bgfx,bkaradzic/bgfx,mendsley/bgfx,jdryg/bgfx,bkaradzic/bgfx,mendsley/bgfx,emoon/bgfx,jdryg/bgfx,bkaradzic/bgfx,bkaradzic/bgfx,mendsley/bgfx,jpcy/bgfx,LWJGL-CI/bgfx