#3170 added checks for nan in the shader code.
The "COMPAT" shader code is intended for running on OpenGL 2.1 and OpenGL ES 2.0, and in those versions, isnan is not defined. Fix by replacing isnan with a self comparison.

Notes:

1. On my system, even when forcing it to use the "COMPAT" shader code, the use of isnan doesn't trigger an error. An error is only triggered if the GLSL version is explicitly specified as #version 120. i.e. the shader compiler can be permissive and support functions that appear in newer GLSL versions.
2. It was not possible to test the shader errors on the CI, because even if illegal keywords were put into the shader code, the CI did not detect that there was a shader compilation error.
3. Windows and Linux are not going to be affected by this bug because they will take the modern shader code path.
4. macOS is possibly affected by this bug as its default OpenGL version is 2.1.

A simple script to exercise the shader code.

import numpy as np
import pyqtgraph as pg

pg.setConfigOptions(useOpenGL=True, enableExperimental=True, background='w')

Z = np.linspace(0, 1, 12).reshape((3, 4))
Z[1,1] = np.nan

pg.mkQApp()
win = pg.PlotWidget()
pcmi = pg.PColorMeshItem(Z)
win.addItem(pcmi)
win.show()
pg.exec()