mesa: flush and wait after creating a fallback texture

Fixes non-deterministic failures in
dEQP-EGL.functional.sharing.gles2.multithread.simple_egl_sync.images.texture_source.teximage2d_render
and others in dEQP-EGL.functional.sharing.gles2.multithread.*

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/texobj.c

@@ -1051,6 +1051,11 @@ _mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex)
       assert(texObj->_MipmapComplete);
 
       ctx->Shared->FallbackTex[tex] = texObj;
+
+      /* Complete the driver's operation in case another context will also
+       * use the same fallback texture. */
+      if (ctx->Driver.Finish)
+         ctx->Driver.Finish(ctx);
    }
    return ctx->Shared->FallbackTex[tex];
 }