vrend: Bind framebuffer when creating context

This make sure we have a valid framebuffer in current context.
Some GPU driver like MALI is buggy and will crash on glEndQuery with targets
like GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN if no valid framebuffer in
current context.

Signed-off-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/vrend_renderer.c

@@ -10309,6 +10309,7 @@ void vrend_renderer_create_sub_ctx(struct vrend_context *ctx, int sub_ctx_id)
    }
 
    glGenFramebuffers(1, &sub->fb_id);
+   glBindFramebuffer(GL_FRAMEBUFFER, sub->fb_id);
    glGenFramebuffers(2, sub->blit_fb_ids);
 
    list_inithead(&sub->programs);