VirtGL: Fix memory leak in eglDestroyPixmap

VirtGL: Fix memory leak in eglDestroyPixmap

The code in qemu for glXDestroyPixmap make some verifications that fails :
if ( qsurface &&
     qsurface != process->current_state->current_qsurface &&
     qsurface->glstate == NULL &&
     qsurface->type == SURFACE_PIXMAP)
    /* free  image data */

We can see that before freeing surface : qemu checks that it is pixmap, it is
not current and it is not bound.

I added some traces and this is a sample from output :
[trace:qemu:opengl][2225]> glXDestroyPixmap
[trace:qemu:opengl]glXDestroyPixmap: 0x7f6898fff150
[trace:qemu:opengl]process->current_state->current_qsurface: (nil)
[trace:qemu:opengl]qsurface->glstate: 0x7f689801eef0
[trace:qemu:opengl]qsurface->type: 1, SURFACE_PIXMAP: 1
[trace:qemu:opengl]process->current_state: 0x1627680, qsurface->glstate:
0x7f689801eef0

We can interpret this as : the application requested to free the surface, but
it was still bound in the context.

The two following reasons suggest to resolve the leak by forcing unbinding the
image from qemu.
* Having a destroyed image in a context doesn't really make sense,
* In this use case it is likely that the context will be destroyed soon after
the pixmap.