mgc::ShmBuffer: Fix texture setup synchronisation.

Because we're on an `EGLContextExecutor` we're probably on a different
thread to where the GL state is going to be used, *and* we don't
have implicit flushing with `eglMakeCurrent` happening (because
the context just stays current on the `EGLContextExecutor`.

If the GL implementation has per-thread execution queues (for example,
amdgpu by default), this might mean that the texture setup commands
aren't visible to command stream that's actually using the
texture.

Explicitly `glFlush()` after our texture setup, to ensure
these commands are visible to any `EGLContext` that might
need them.

Author: RAOF

src/platforms/common/server/shm_buffer.cpp

@@ -122,6 +122,16 @@ auto get_tex_id_on_context(mgc::EGLContextExecutor& egl_executor) -> std::shared
 
             // ...and then restore the previous GL state.
             glBindTexture(GL_TEXTURE_2D, static_cast<GLuint>(previous_texture));
+
+            /* We need these commands to be in the driver command stream before
+             * anything which uses them can be executed (which will likely be
+             * on another thread, in another EGLContext).
+             *
+             * glFlush() before the promise synchronisation point will ensure
+             * those commands are visible to the driver before we try and use
+             * their results.
+             */
+            glFlush();
             tex_promise->set_value(tex);
         });
     return tex_promise->get_future();