This allows the paint server code to be unit tested in later versions. There are some limitations
to this refactoring including use of virtual functions instead of casting so we can use templates
in renderers.

ll SPObjects must implement their virtual functions, so the base classes all return empty objects
where possible and this pushed the design in a certain direction for some calls.

 * Move sp-gradient spread, units and vector to sp-paint-server-data.h
 * Move mesh gradient patches data from display/drawing-paintserver to sp-paint-server-data
 * Allow SPPaintServer to be used as a template class in a renderer by providing data from sp-paint-server-data in a consistant way from the base class using virtual functions.
 * Add getPaintType and enum class to decide what type of paint server we have instead of cast
 * Add SPGradient virtual for gathering geometric gradient data for Lienar and Radial gradients
 * Add SPMeshGradient virtual for gathering mesh patches data for the Mesh gradient
 * Pack gradient geometric data into SPGradientVector and return in getGradientVector
 * Harmonize const and API between Mesh Gradients and Linear/Radial Gradients
 * Move add create_drawing_paintserver calls to display/drawing-paintserver in a single call
 * Remove `mg` MeshGradient pointer from MeshArray and remove back-linking. This indicates bad
   API design for some of these calls as the functions used mg and had little to do with the MeshArray
   but for now we just push the MeshGradient object back in as an argument.
 * Change drawing-paintserver constructors to take paint-server-data objects instead of individual arguments.

Remove remaining uses of SCALE24_TO_FLOAT and FROM_FLOAT macros which were only
used for SPIFloat and can easily be folded into the object itself.

Harmonize the default data type use with SPIEnum, SPILength and SPIFontSize
this allows the rendering engine nr-style.cpp to use the right rendering value
which is always the computed value and not need to think about the internal
implementation of the style variable between `value`, `computed` and will allow
these variable to be made private in the future.

All uses are explicit to rule out mistaken variable type use.