Fix undefined behavior from array “shape-punning”

[musicinmybrain]

In stb_voxel_render.h, there were three cases where a 2D array of dimension [X][Y] was iterated as a 1D array of dimension [1][X*Y]. While this is clever and is correct in terms of the actual memory layout, a second index outside the corresponding dimension ([i][j], j >= Y]) actually produces undefined behavior and gives the compiler freedom to do all sorts of terrible things.

The same thing happens in stb_tilemap_editor.h, tests/caveview/cave_mesher.c, and tests/resample_test.cpp.

Prior to this commit, a compiler warning regarding the undefined behavior appears on gcc 11.2.1 for at least some of these cases when the tests are compiled with -Waggressive-loop-optimizations (included in -Wall). For example:

test_c_compilation.c: In function ‘stbvox_make_mesh_for_block_with_geo’:
../stb_voxel_render.h:3134:46: warning: iteration 4 invokes undefined behavior [-Waggressive-loop-optimizations]
 3134 |          int vert = stbvox_vertex_selector[0][i];
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~^~~
../stb_voxel_render.h:3133:19: note: within this loop
 3133 |       for (i=0; i < 6*4; ++i) {
      |                 ~~^~~~~
../stb_voxel_render.h:3280:49: warning: iteration 4 invokes undefined behavior [-Waggressive-loop-optimizations]
 3280 |             int vert = stbvox_vertex_selector[0][i];
      |                        ~~~~~~~~~~~~~~~~~~~~~~~~~^~~

This commit fixes the undefined behavior by iterating these 2D arrays with the conventional nested loops.

(This is an important but conceptually trivial fix, originally suggested by Jerry James. I don’t believe it warrants adding anyone to the list of contributors.)