show pointer using opengl texture overlay

[Antoine Martin]

The pointer overlay feature is used when sharing is enabled or when connecting to a shadow server.
It allows the user to see where the pointer actually is.

I thought it would be easy to paint it with opengl... I was wrong.

Changesets:

• r15562: expose shadow pointer position (sharing already enabled this for regular servers)
• r15563: send cursor shape (adds polling for X11 shadow servers)
• r15564: refactoring, decode PNG cursors in packet handler
• r15565: don't bother polling stuff when no-one is listening!
• r15567 + r15568 (backwards compatibility fixup): clients can request the default cursor pixels (which are not sent in cursor packets), opengl refactoring, log cursor changes, etc
• r15569 + r15570 + r15575: opengl cleanups and improvements

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Remaining problem: with XPRA_OPENGL_TEXTURE_CURSOR=1, the cursor is shown but it doesn't get blended with the PBO.
Changing the GL_TEXTURE_ENV_MODE to use blending looks like it might work... but this affects the whole texture unit and messes up the rendering of the PBO.
Using another texture unit via glActiveTexture(GL_TEXTURE1) makes it render as a white rectangle instead.

[Antoine Martin]

Horrid fix in r15576, will follow up with #1497 instead.

On the plus side, the opengl code is now cleaner and more readable.