Use idiv in flat indexing.

[annxingyuan]

This fixes the posenet demo for WeChat miniapps.

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This change is 

[dsmilkov]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @annxingyuan and @dsmilkov)

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src/backends/webgl/encode_matrix_gpu.ts, line 48 at r1 (raw file):

        int offset = imod(flatIndex, 4);

        flatIndex = idiv(flatIndex, 4, 1.);

if you look at the definition of idiv:

int idiv(int a, int b, float sign) {
    int res = a / b;
    int mod = a % b;
    if (sign < 0. && mod != 0) {
      res -= 1;
    }
    return res;
  }

and given that the sign is 1, it effectively translates to:

int idiv(int a, int b, float sign) {
    int res = a / b;
    return res;
 }

so there is only two possibilities:

• either the /= operator is the culprit
• or the 4 needs to be explicitly wrapped as an int, that is flatIndex /= int(4);

Can you test those two possibilities when you get a chance? It would be good to know which one it is so we can defend other parts of the glsl code as well.

[pyu10055]

@dsmilkov @annxingyuan good analysis, I just verified it is the /= problem.
flatIndex = flatIndex / 4 works as well.