The border and color shaders - the ones that do AA - now multiply their
coordinates by the scale factor, which gives them better rounding
capabilities.
This in particular improves the case where they are used in fractional
scaling situations, where the scale is defined at the root element.
This introduces the rect object and adds a rect_distance() and
rect_coverage() function.
_distance() returns the signed distance tp the rectangle.
_coverage() returns the coverage of a pixel centered at that position.
Note that the pixel size is computed using dFdx/dFdy.
Move the resources of each renderer to its subdirectory.
We've previously done that for the ngl renderer, but it
is better to be consistent and do it for all the renderers.
