[Feature] Handle large plots with 16-bit index

[brenocq]

Feature description
This feature addresses the common "16-bit index overflow" issue in Dear ImGui. By default, ImDrawIdx is a short, limiting draw calls to 65535 vertices. High-density 3D surfaces and meshes easily exceed this limit, leading to visual glitches for users who haven't manually enabled 32-bit indices. ImPlot3D should internally manage vertex counts and automatically split draw commands to ensure stability across all configurations.

This issue was also reported here: #164

Tasks

• Split rendering into multiple draw calls if needed
• Create a stress-test demo with a 500×500 surface plot (250k vertices)

[brenocq]

Below is an example of visual artifacts because the number of indices exceeded 65535. In this case, there were not enough indices to plot the lines (each line is 2 triangles).