Tables: Reworked padding/spacing/width.

All widths are stored without padding.
Decorelate padding from presence of border.
Added ImGuiTableFlags_Pad/NoPad flags.
Added demo.
Merge StartXHeaders and StartXRows into StartX.
Distinguish CellSpacingX1 and CellSpacingX2 for clarity and to avoid loss of width on non-even spacing values.
4 files changed