Expose a function for reading the bitmap of rendered content (windowed mode)

[cztomczak]

In the past in CEF 1 there was a function in CEF 1 that allowed to get the bitmap of rendered content. Such feature was removed from upstream CEF, however it would be nice to have it back in cefpython. There are various native APIs on Windows/Linux/Mac that allow to get a screenshot of window's content. When using off-screen rendering you are already provided with a bitmap in RenderHandler.OnPaint. This issue is to expose such functionality also in windowed mode.

[cztomczak]

Browser.GetImage method is availably only for Linux. If you are interested in exposing this method also on Windows or Mac then PRs are welcome.

[cztomczak]

Xlib can return browser image in the following formats: XYBitmap, XYPixmap, ZPixmap. I am setting ZPixmap and then converting it to RGB format in Cython which can slow down things a bit. If you want the xlib pixmap/bitmap (for performance reasons) then it would be required to introduce a new param to GetImage, so that you can choose format.