Simplify PlotSurface API

[brenocq]

Idea from @fecaltransplant in #3

Also, in my data I have fixed x and y value ranges. Would it be possible to enable that option in the plot surface to ease the entry of the x and y value ranges (without repeating them over and over in their respective arrays?)

Currently, ImPlot3D requires users to define explicit X, Y, and Z coordinates for each point in a surface plot, offering great flexibility but adding unnecessary overhead for structured data. In many use cases, the data naturally lies on a regular NxM grid, meaning:

• The X values are shared across N columns.
• The Y values are shared across M rows.
• The Z values form an N × M matrix representing the surface heights.

To improve usability and efficiency, ImPlot3D should support an alternative API where users can specify:

• N unique X values.
• M unique Y values.
• N*M Z values.