[glcanvas] save as PNG/JPG/BMP image does not work for first time

[linev]

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Description

When GL canvas is enabled, first stored image is black.
But second time storage works fine.

Reproducer

root --web=off -l
root [0] gStyle->SetCanvasPreferGL(true);
root [1] .x $ROOTSYS/tutorials/visualisation/graphics/tmathtext.C 

Macro create PNG and PS image. PNG is just black, PS is ok.
If store canvas second time, it is fine:

root [2] c1->SaveAs("c2.png")

ROOT version

Checked master, 6.40, 6.38, most probably all previous branches

Installation method

Build from source

Operating system

Linux

Additional context

When second image created, it looks fine.
PS output works from beginning

[bellenot]

FYI it works fine on Windows

[linev]

It is something simple - but I have no time to debug it now.