screencasting: fix vertical-mirror issues when going through grabTexture

Ensures the texture we get from non-drm backends is properly marked as yInverted so it renders consistently when on the GL-specific paths. Never use GLTexture::yTexture to consider mirroring from grabTexture. It's not necessary, we just need to do it in the GLES case and we always need to do it in that case. We got there because:

• we mostly test with dmabuf
• having the backends behave differently made it seem like it depended on the property.

Test plan:

• drm (with and without dmabuf): full session with recordme
• windowed (with & without dmabuf): dbus-launch kwin_wayland --socket potato --width 1440 --height 720 --virtual --exit-with-session recordme plasmashell
• virtual: dbus-launch kwin_wayland --socket potato --width 1440 --height 720 --virtual --exit-with-session selenium-webdriver-at-spi-recorder plasmashell; killall selenium-webdriver-at-spi-recorder

Marked as draft because there's stuff I still need to understand to consider it as good