Following #2111, this PR tries to eliminate PyOpenGL automatic array conversions in the OpenGL items by making the conversions explicitly. The places where conversion was taking place was discovered by setting OpenGL.ERROR_ON_COPY = True in the examples and running them. With this PR, all included OpenGL examples are able to run without error with the above attribute set.

GLImageItem and GLVolumeItem deserve a mention. Pyqtgraph uses Fortran-major axes and thus transposes the texture data before uploading them. GLImageItem was fixed to avoid the transpose by changing the texture coordinate code. The texture coordinate code of GLVolumeItem was too complicated to fix, thus a memory copy is forced.

The script below was used to verify that GLImageItem draws the same before and after this PR.

import pyqtgraph as pg
import pyqtgraph.opengl as gl
import numpy as np
import scipy.misc

pg.mkQApp()

glv = gl.GLViewWidget()
glv.setCameraParams(elevation=90, azimuth=0)
# X points down, Y points right
glv.show()

image = scipy.misc.face()
tex, _ = pg.makeRGBA(image)
glii = gl.GLImageItem(tex)
s = 1/128
glii.scale(s, s, 1)
w, h = tex.shape[:2]    # GLImageItem uses transposed coords?
w *= s
h *= s
glii.translate(-w/2, -h/2, 0)
glv.addItem(glii)

spots = np.array([[-w/2, -h/2, 0, 1, 0, 0, 0.5],
                  [-w/2,  h/2, 0, 0, 1, 0, 0.5],
                  [ w/2,  h/2, 0, 0, 0, 1, 0.5],
                  [ w/2, -h/2, 0, 0, 1, 1, 0.5]])
glspi = gl.GLScatterPlotItem(pos=spots[:,:3], size=50, color=spots[:,3:])
glv.addItem(glspi)

pg.exec()

This PR may fix #825.