Fix qmlglsrc vertical flip matrix
commit ae80fa47
qmlglsrc: read from the back buffer when use-default-fbo = TRUE
glReadBuffer(GL_COLOR_ATTACHMENT0) on the default framebuffer (0) is
invalid GL API usage and would result in a GL error being thrown.
commit 7f1e1d0d
qmlglsrc: fix vertical flip matrix
Some time ago libgstgl defined the majorness of matrices it uses.
The majorness used by qmlglsrc was incompatible with the libgstgl.