fix painting cursor with opengl

[totaam]

Follow up from #1496, see r15576.

We copy the pixels one at a time which is horrid.
Someone who understands opengl should fix the texture code instead..

[totaam]

For reference, the current (broken) code can be enabled using:

XPRA_OPENGL_TEXTURE_CURSOR=1 xpra attach ...

And the actual painting code can be seen by looking for TEXTURE_CURSOR in /browser/xpra/trunk/src/xpra/client/gl/gl_window_backing_base.py.

[totaam]

Well, I'll be damned!

Following the precious advice that glEnablei(GL_BLEND) was invalid, I removed it in r23265. (after wrongly trying to replace it by glEnable(GL_BLEND) in r23263, which quickly got reverted in r23264 as painting was broken)

r23266 + r23267 does replace it though, but only for the cursor paint code, and now it seems to work!
So r23269 enables it by default.

I am keeping this ticket open, because this is going to need a lot more testing.
Already tested:

• win32 shadow server
• seamless X11 transparent windows

[totaam]

r23270 also disables GL_BLEND once we're done painting the cursor

[totaam]

Works great.