Tool overlay and cursor shapes

[LukasT.dev@gmail.com]

Hi, 

I would like to work on tool overlays (I consider tool overlay line preview, 
circle preview, brush outline) and fix related bugs. 

I'm waiting whether somebody approve my commit which uses QImage as buffer in 
canvas although we have many buffers in the render chain...At least my patch 
would help me to fix some issues (I could use various compositing modes, e.g. 
XOR, without fear that some platform or X11 would not work correctly)

Related bugs:
* When you change the cursor shape in Krita, it is changed after you change 
the tool from freehand to some other tool and back to freehand.

* Brush outlines are invisible on black surfaces. Solution is composition mode 
ala XOR or black-and-white outline of the tool. This was discussed on IRC with 
boemann and Cyrille and I tested various approaches for this so just document 
your opinion here please.

[1] - black and white, bad version, tool overlays should not be scaled
[2] - correct the bad version ^
[3] - no anti-aliasing, b&w again
[4] - anti-aliasing, view on gradient
[5] - dashed b&w version

I would prefer some intelligent XOR instead of black&white versions. When the 
under-laying pixel has brightness less then 50 use black pixel, white 
otherwise...

Pure XOR has some "127 problem" which boemann was talking about and this can 
be partially solved or moved when you use color like gimp uses -- 
RGB(128,255,128).

* When brush outline is selected, only freehand provides something, other 
tools are in mode No Cursor or something.

Your opinions?

Lukas

[1] http://imagebin.ca/view/dD9wQP7E.html
[2] http://imagebin.ca/view/1hrbEwJS.html
[3] http://imagebin.ca/view/IgZEvtN4.html
[4] http://imagebin.ca/view/sGPG5uH.html
[5] http://imagebin.ca/view/4PXEbJ9H.html