https://bugs.freedesktop.org/show_bug.cgi?id=94595
--- Comment #8 from Nicolai Hähnle nhaehnle@gmail.com --- There is a patch at https://patchwork.freedesktop.org/patch/77510/ which fixes your test case (and should fix format casting with render-to-texture more generally).
Let us know if this fixes your SRGB-related rendering problems.