glx: avoid memory leak when using indirect rendering

When multiple processes are using GL with indirect rendering a race
condition can make drawables refcount never drop to zero.

This situation could happen when there are many X clients using indirect
GLX:

1 - client1: calls glXMakeCurrent

2 - client2: calls glXMakeCurrent
This is the first context switch for this client. So old_context_tag=0

3 - client1: calls glXRender
For the client, its context is already current.
For the server side lastGLContext points to client2's context.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Guilherme Quentel Melo <gqmelo@gmail.com>

glx/glxext.c

@@ -466,6 +466,12 @@ __glXForceCurrent(__GLXclientState * cl, GLXContextTag tag, int *error)
 
     /* Make this context the current one for the GL. */
     if (!cx->isDirect) {
+        /*
+         * If it is being forced, it means that this context was already made
+         * current. So it cannot just be made current again without decrementing
+         * refcount's
+         */
+        (*cx->loseCurrent) (cx);
         lastGLContext = cx;
         if (!(*cx->makeCurrent) (cx)) {
             /* Bind failed, and set the error code.  Bummer */