Slightly rewrite the convolve method

[LaurenzV]

This is a follow-up to #1601.

As was mentioned there, we seemingly cannot use indices to index the vector, otherwise it fails on Windows. Therefore, we changed it so the vector is first converted into an array. However, this now leads to issues on certain Mali GPUs (in this case tested on a Realme 8, but I saw other phones affected by this as well). A shader as the following:

#version 300 es
precision highp float;
precision highp int;

out vec4 outColor;

void main() {
  float weight = 0.25;
  float weights_arr[1] = float[1](weight);
  outColor = vec4(weights_arr[0], 0.0, 0.0, 1.0);
}

unfortunately fails to compile for many phones:

First zero-initializing and then assigning each field seems to work fine:

float weights_arr[1] = float[1](0.0);
weights_arr[0] = weight;

Therefore, we need to use this approach as well to make it work on those phones.