Xpra: Ticket #1769: Getting Occasional OpenGL Tracebacks

My server and client are both Fedora 26 machines running trunk r18446 built from source.

My client machine has an Nvidia GPU (a GTX 745 to be specific), and occasionally I will get this traceback (usually while running Firefox - as far as I can tell it's not specific to any sites, I've gotten it on Wikipedia before). Normally I frown upon writing "Weird errors in the logs" tickets without thoroughly understanding what's going on first, but OpenGL isn't my forte and usually when I see this traceback in the log the paints go a little weird. The last couple times I saw it, Firefox stopped painting altogether until I re-attached.

Here's the traceback:

2018-02-16 11:21:28,675  flush=2, image=ImageWrapper(YUV420P:(0, 0, 1244, 1, 24):PACKED), coords=(0, 325, 1244, 1), size=1244x1
2018-02-16 12:49:45,405 Error painting planar update
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/xpra/client/gl/gl_window_backing_base.py", line 988, in gl_paint_planar
    self.update_planar_textures(x, y, enc_width, enc_height, img, pixel_format, scaling=(enc_width!=width or enc_height!=height))
  File "/usr/lib64/python2.7/site-packages/xpra/client/gl/gl_window_backing_base.py", line 1054, in update_planar_textures
    glTexSubImage2D(target, 0, 0, 0, width//div_w, height//div_h, GL_LUMINANCE, GL_UNSIGNED_BYTE, pixel_data)
  File "latebind.pyx", line 32, in OpenGL_accelerate.latebind.LateBind.__call__ (src/latebind.c:989)
  File "wrapper.pyx", line 318, in OpenGL_accelerate.wrapper.Wrapper.__call__ (src/wrapper.c:6561)
GLError: GLError(
	err = 1281,
	description = 'invalid value',
	baseOperation = glTexSubImage2D,
	pyArgs = (
		GL_TEXTURE_RECTANGLE_ARB,
		0,
		0,
		0,
		622,
		0,
		GL_LUMINANCE,
		GL_UNSIGNED_BYTE,
		<memory at 0x7f914813c938>,
	),
	cArgs = (
		GL_TEXTURE_RECTANGLE_ARB,
		0,
		0,
		0,
		622,
		0,
		GL_LUMINANCE,
		GL_UNSIGNED_BYTE,
		<memory at 0x7f914813c938>,
	),
	cArguments = (
		GL_TEXTURE_RECTANGLE_ARB,
		0,
		0,
		0,
		622,
		0,
		GL_LUMINANCE,
		GL_UNSIGNED_BYTE,
		<memory at 0x7f914813c938>,
	)
)
2018-02-16 12:49:45,406  flush=0, image=ImageWrapper(YUV420P:(0, 0, 1244, 1, 24):PACKED), coords=(0, 547, 1244, 1), size=1244x1