renderer/gles: disable fence export after runtime failure

[krissedout]

Description

• disable ExportFence after EGLFence::create fails at runtime
• use the existing synchronized glFinish fallback for the failed frame and subsequent frames
• avoid retrying a broken eglCreateSync call on every rendered frame

Some context:
GlesRenderer enables ExportFence when the required GL extension is advertised. Some drivers advertise support but still fail when Smithay attempts to create the EGL fence, occasionally without setting an EGL error.

export_sync_point currently returns None after that failure but retains the capability, causing the same failing EGL call and warning to repeat every frame.

Once fence creation fails for a renderer context, this change removes ExportFence from that renderer’s capabilities. Smithay then follows its existing glFinish fallback path, preserving synchronization and correctness without repeated EGL calls.

Checklist

• I agree to smithay's Developer Certificate of Origin.