shaders/sampling: fill the lut padding

This ensures that we don't interpolate lut values with garbage values
when row_size is not multiple of 4. Avoid UB that produces incorrect
values.

This commit fixes the fast path of orthogonal scaling.

Fixes: mpv-player/mpv#13998

src/shaders/sampling.c

@@ -919,12 +919,16 @@ static void fill_ortho_lut(void *data, const struct sh_lut_params *params)
             const float *weights = filt->weights + n * filt->row_stride;
             float *row = (float *) data + n * filt->row_stride;
             pl_assert(filt->row_size % 2 == 0);
-            for (int i = 0; i < filt->row_size; i += 2) {
+            int i = 0;
+            for (; i < filt->row_size; i += 2) {
                 const float w0 = weights[i], w1 = weights[i+1];
                 assert(w0 + w1 >= 0.0f);
                 row[i] = w0 + w1;
                 row[i+1] = w1 / (w0 + w1);
             }
+            pl_assert(filt->params.row_stride_align == 4); // always 4 components
+            for (; i < filt->row_stride; i++)
+                row[i] = i >= 4 ? row[i - 4] : 0;
         }
     } else {
         size_t entries = SCALER_LUT_SIZE * filt->row_stride;