PColorMeshItem is unusable when a non-small mesh is used.
Even panning a static PColorMeshItem is very slow. (i.e. just redrawing the rendered QPicture is slow)

Following the example of the recent OpenGL code updates to PlotCurveItem, we can make PColorMeshItem usable by rendering using OpenGL.

An example to demonstrate the speed.
We can use the peegee icons of various sizes to test out the performance.
On my laptop, peegee_256px.png without OpenGL gets a dismal 5 fps; whereas with OpenGL, peegee_512@2x.png hits 30 fps. i.e. an improvement of (1024/256)**2 * 30/5 == 96x

import sys

import numpy as np

import pyqtgraph as pg
from pyqtgraph.Qt import QtCore, QtGui
from pyqtgraph.examples.utils import FrameCounter

qimage = QtGui.QImage(sys.argv[1])
qimage.convertTo(QtGui.QImage.Format.Format_Grayscale8)
z = pg.functions.ndarray_from_qimage(qimage)


Z = z
ny, nx = z.shape
x = np.linspace(0, nx, nx+1, endpoint=True)
y = np.linspace(0, ny, ny+1, endpoint=True)
X, Y = np.meshgrid(x, y)
X += (ny * 0.025) * np.sin(2*np.pi*2*np.linspace(0, 1, ny+1))[:, np.newaxis]
Y += (nx * 0.025) * np.sin(2*np.pi*2*np.linspace(0, 1, nx+1))

pg.setConfigOptions(useOpenGL=True, enableExperimental=True)

pg.mkQApp()
win = pg.PlotWidget()
win.invertY(True)
cmap = pg.colormap.ColorMap(None, [0.0, 1.0])
pcmi = pg.PColorMeshItem(X, Y, Z, colorMap=cmap)
win.addItem(pcmi)
win.setTitle('')
win.show()

cnt = 0
def update():
    global cnt
    cnt = (cnt + 1) % z.shape[1]
    Z = np.roll(z, cnt, axis=1)
    pcmi.setData(None, None, Z)
    framecnt.update()

timer = QtCore.QTimer()
timer.timeout.connect(update)
timer.start()

framecnt = FrameCounter()
framecnt.sigFpsUpdate.connect(lambda fps: win.setTitle(f'{fps:.1f} fps'))

pg.exec()

An addition to the PColorMeshItem API. A use-case that I have is that the mesh (XY) is fixed and only the Z values are varying. I have overloaded the setData(None, None, Z) to mean that the existing X, Y should be re-used. This allows to not waste time uploading vertices that have not changed.
(The converse also works: setData(X, Y, None) means that Z should be re-used; although I don't have such a use-case)

One wart is that on termination, the following error message is shown:

QOpenGLTexturePrivate::destroy() called without a current context.
Texture has not been destroyed

This message doesn't show up on a pure QOpenGLWidget implementation. (as opposed to this PR's doing OpenGL painting within a QGraphicsView)