[Piglit] New test for partial blits after a buffer swap

[Eric Anholt]

Federico Mena Quintero <federico.mena.lists at gmail.com> writes:

> Hello, everyone,
>
> Attached is a new test for piglit which exposes a bug in Mesa's
> software rendering (and another bug in hardware rendering, but that's
> not its main purpose).
>
> The bug is as follows.  With software rendering, after doing a buffer
> swap, glBlitFrameBufferEXT() appears to to copy the whole framebuffer
> instead of just the specified region.  This breaks clutter and cogl,
> since they keep track of a dirty region themselves, and they use blits
> instead of full buffer swaps to avoid updating the whole display on
> every frame unnecessarily.
>
> What is happening is actually a bit more complicated.
> glBlitFrameBufferEXT()'s basic machinery works correctly, but if there
> has been a buffer swap before it, the following happens:

Hah.  Apparently the problem is you're hitting this line of
platform_x11.c in EGL:

   /* FIXME: Does EGL support front buffer rendering at all? */
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