Cairo: Explore XLib backend

[tinchodias]

Related to #92 and #91. There seems to exist a x11 backend for cairo, that allows user to create a platform-specific surface that I guess may be optimized, at least, for some scenarios: https://www.cairographics.org/manual/cairo-XLib-Surfaces.html

This is a demo: https://github.com/msteinert/cairo-demo/blob/master/cairo-demo-x11.c

To create a xlib surface, you need to provide a X's Display, a Drawable (a Window or Pixmap) and a Visual (a kind of image format). How to get them? SDL2 should provide it via the info structs.

As a preliminar test, I created a LibX11 as a duplicate of LibC but defined these methods:

unixLibraryName
	^ 'libX11.so'
defaultVisualOnDisplay: displayPointer screenNumber: screenNumber
	^ self ffiCall: #("Visual"void *XDefaultVisual("Display"void *displayPointer, int screenNumber))
displayWidthOf: displayPointer screenNumber: screenNumber
	^ self ffiCall: #(int XDisplayWidth("Display"void *displayPointer, int screenNumber))
displayHeightOf: displayPointer screenNumber: screenNumber
	^ self ffiCall: #(int XDisplayHeight("Display"void *displayPointer, int screenNumber))

The width and height were right, when executed on a Workspace:

w := SDL_Window allInstances detect: [ :each | each isNull not ].
wmInfo := SDL_SysWMinfo new version: SDL_Version bindingVersion.
w getWMInfo: wmInfo.
window := wmInfo info x11 window.
display := wmInfo info x11 display.
LibX11 uniqueInstance displayWidthOf: display screenNumber: 0. "1366"
LibX11 uniqueInstance displayHeightOf: display screenNumber: 0. "768"
visual := LibX11 uniqueInstance defaultVisualOnDisplay: display screenNumber: 0.

This should be enough to try the cairo-xlib surface create function.

[tinchodias]

I added a draft demo of the surface working in a branch that has "93-" as prefix.

It worked on my Debian with wayland. It draws directly to the main window. It doesn't make sense to invest time on this right now. To draw on a separate image (not the window), we should implement 2 functions: https://tronche.com/gui/x/xlib/pixmap-and-cursor/pixmap.html