OpenGL/computegles31 example crashed on raspberry pi 4 due to big compute worgroup size

With introduction of OpenGL ES 3.1(in Mesa 2019.3-devel) on Raspberry Pi 4/VideoCoreVI hardware it's possible to run Compute Shaders, including computegles31 example from qtbase/opengl.
Unfortunately, Rpi4/VideoCoreVI hardware is quite limited, in particular MAX_COMPUTE_WORK_GROUP_INVOCATIONS is only 256(16*16), while computegles31 example required 1024(32x32).

Changing COMPUTEPATCHSIZE parameter(both in GLES shader and ++ code) from 32 to 16 make it possible to run computegles31 example on RPi4/VideoCoreVI (and may be some other hardware, like Vivante GPU).

Technically it's a really minor bug, while it can negatively affect a new Qt users/students with limited knowledge about GL/GLES/Compute shaders.