linux-capture: Fix capturing on software rasterization setups

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The GLX_EXT_texture_from_pixmap spec says:

The contents of the texture after the drawable has been bound are defined
as the result of all rendering that has completed before the call to
glXBindTexImageEXT.  In other words, the results of any operation which
has caused damage on the drawable prior to the glXBindTexImageEXT call
will be represented in the texture.

Rendering to the drawable while it is bound to a texture will leave the
contents of the texture in an undefined state.  However, no
synchronization between rendering and texturing is done by GLX.  It is
the application's responsibility to implement any synchronization
required.

In practice, on most systems with a GPU this kept a directly binding to
the framebuffer, which made it work with the previous code. However,
on software rasterization setups using llvmpipe, a blit was done inside
the call to glXBindTexImageEXT. This was the cause of a notoriously
ignored bug where the captured image would "freeze" until the source
was reconfigured.

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