qt6: if def newer symbosl in QRhiTexture
version 6.4 added QRhiTexture::RGB10A2 but we depend on an older version of qt in meson, and we can keep compiling with older Qt6 versions still.
version 6.4 added QRhiTexture::RGB10A2 but we depend on an older version of qt in meson, and we can keep compiling with older Qt6 versions still.