Cleanup SPIFloat and default values in styles

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.