_cogl_texture_flush_journal_rendering is optimized to avoid glFlushing
the driver. That's fine for most cases but insufficient if it's followed
by a driver call like
So in some cases we were generating mipmaps from an uninitialized texture.
This fixes mipmap corruption observed on Xilinx (Mali): https://github.com/Xilinx/mali-userspace-binaries/issues/3
Related to: #2354 (closed)