mesa: drop current draw/read buffer when ctx is released

This fixes a problem seen with gallium drivers vs android wallpaper.
Basically, what happens is:

   EGLSurface tmpSurface = mEgl.eglCreatePbufferSurface(mEglDisplay, mEglConfig, attribs);
   mEgl.eglMakeCurrent(mEglDisplay, tmpSurface, tmpSurface, mEglContext);

   int[] maxSize = new int[1];
   Rect frame = surfaceHolder.getSurfaceFrame();
   glGetIntegerv(GL_MAX_TEXTURE_SIZE, maxSize, 0);

   mEgl.eglMakeCurrent(mEglDisplay, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);
   mEgl.eglDestroySurface(mEglDisplay, tmpSurface);

   ... check maxSize vs frame size and bail if needed ...

   mEglSurface = mEgl.eglCreateWindowSurface(mEglDisplay, mEglConfig, surfaceHolder, null);
   ... error checking ...
   mEgl.eglMakeCurrent(mEglDisplay, mEglSurface, mEglSurface, mEglContext);

When the window-surface is created, it ends up with the same ptr address
as the recently freed tmpSurface pbuffer surface.  Which after many
levels of indirection, results in st_framebuffer_validate() ending up with
the same/old framebuffer object, and in the end never calling the
DRIimageLoaderExtension::getBuffers().  Then in droid_swap_buffers(), the
dri2_surf is still the old pbuffer surface (with dri2_surf->buffer being
NULL, obviously, so when wallpaper app calls eglSwapBuffers() nothing
gets enqueued to the compositor).  Resulting in a black/blank background
layer.

Note that at the EGL layer, when the context is unbound, EGL drops it's
references to the draw and read buffer as well.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Robert Foss <robert.foss@collabora.com>
Acked-by: Tapani Pälli <tapani.palli@intel.com>

src/mesa/main/context.c

@@ -1651,6 +1651,10 @@ _mesa_make_current( struct gl_context *newCtx,
 
    if (!newCtx) {
       _glapi_set_dispatch(NULL);  /* none current */
+      if (curCtx) {
+         _mesa_reference_framebuffer(&curCtx->WinSysDrawBuffer, NULL);
+         _mesa_reference_framebuffer(&curCtx->WinSysReadBuffer, NULL);
+      }
    }
    else {
       _glapi_set_dispatch(newCtx->CurrentDispatch);