Enable 32-bit vertex indices

[ggerganov]

ref #5

• This change enables 32-bit vertex indices in Dear ImGui and also makes the WebGL renderer use 32-bit vertex arrays
• Your browser must support the OES_element_index_uint extension in order to run this
• After checking out this branch, make sure to remove CMakeCache.txt from your build folder and re-build the project

For now, I don't want to merge this into master yet, because I am not sure how many devices support this WebGL extension. Maybe we need to add some fallback logic if it is not supported.