Relative speedups VS brush size

- Modelling -
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+
|                                   | 2.0  | 4.0  | 8.0  | 16.0 | 32.0 | 64.0 | 128.0 | 256.0 | 512.0 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+
|      PythonSurface, original      | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |  1.00 |  1.00 |  1.00 |
|       GeglSurface, original       | 1.06 | 1.08 | 1.09 | 1.07 | 1.08 | 1.08 |  1.06 |  1.07 |  1.06 |
|      PythonSurface, optimized     | 1.27 | 1.29 | 1.32 | 1.44 | 1.45 | 1.46 |  1.50 |  1.47 |  1.45 |
|  GeglSurface, optimized, 1 thread | 1.56 | 1.57 | 1.60 | 1.69 | 1.70 | 1.70 |  1.71 |  1.70 |  1.68 |
| GeglSurface, optimized, 2 threads | 1.53 | 1.40 | 1.44 | 1.80 | 2.22 | 2.43 |  2.66 |  2.76 |  2.80 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+


- Charcoal -
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+
|                                   | 2.0  | 4.0  | 8.0  | 16.0 | 32.0 | 64.0 | 128.0 | 256.0 | 512.0 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+
|      PythonSurface, original      | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |  1.00 |  1.00 |  1.00 |
|       GeglSurface, original       | 0.68 | 0.85 | 1.04 | 1.07 | 1.16 | 1.18 |  1.14 |  1.11 |  1.11 |
|      PythonSurface, optimized     | 0.96 | 1.00 | 1.10 | 1.22 | 1.32 | 1.36 |  1.40 |  1.37 |  1.36 |
|  GeglSurface, optimized, 1 thread | 1.11 | 1.32 | 1.53 | 1.57 | 1.69 | 1.76 |  1.74 |  1.70 |  1.69 |
| GeglSurface, optimized, 2 threads | 1.05 | 1.18 | 1.37 | 1.59 | 1.92 | 2.10 |  2.42 |  2.77 |  2.76 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+


- Coarse Bulk 2 -
+-----------------------------------+------+------+------+------+------+------+-------+-------+
|                                   | 2.0  | 4.0  | 8.0  | 16.0 | 32.0 | 64.0 | 128.0 | 256.0 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+
|      PythonSurface, original      | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |  1.00 |  1.00 |
|       GeglSurface, original       | 1.08 | 1.46 | 1.55 | 1.34 | 1.23 | 1.17 |  1.16 |  0.98 |
|      PythonSurface, optimized     | 0.99 | 1.02 | 1.12 | 1.21 | 1.25 | 1.30 |  1.32 |  1.30 |
|  GeglSurface, optimized, 1 thread | 1.47 | 1.80 | 1.91 | 1.78 | 1.70 | 1.68 |  1.67 |  1.33 |
| GeglSurface, optimized, 2 threads | 1.48 | 2.05 | 2.37 | 2.57 | 2.53 | 2.61 |  2.68 |  1.77 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+


- Bulk -
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+
|                                   | 2.0  | 4.0  | 8.0  | 16.0 | 32.0 | 64.0 | 128.0 | 256.0 | 512.0 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+
|      PythonSurface, original      | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |  1.00 |  1.00 |  1.00 |
|       GeglSurface, original       | 0.85 | 1.15 | 1.85 | 1.44 | 1.53 | 2.28 |  1.53 |  1.51 |  1.13 |
|      PythonSurface, optimized     | 0.92 | 0.99 | 1.08 | 1.20 | 1.27 | 1.36 |  1.36 |  1.30 |  1.32 |
|  GeglSurface, optimized, 1 thread | 1.34 | 1.54 | 1.65 | 1.69 | 1.74 | 1.75 |  1.71 |  1.65 |  1.61 |
| GeglSurface, optimized, 2 threads | 1.10 | 1.34 | 1.57 | 1.84 | 2.22 | 2.52 |  2.69 |  2.72 |  2.66 |
+-----------------------------------+------+------+------+------+------+------+-------+-------+-------+