Correct "edge" in project with camera

[nighca]

如语音沟通，在 edge 相关的逻辑（包括 BounceOffEdge）中，edge 应当视作 stage 的边缘而不是 viewport/camera 的边缘；详见 https://github.com/goplus/builder/blob/dev/docs/product/camera.zh.md#edge-apis

Originally posted by @nighca in #1130 (comment)