Previously when uploading SHM data we would initialise the texture
with glTexImage2D and NULL data when the buffer is attached. Then if
the GL_EXT_unpack_subimage extension is available we would always use
glTexSubImage2D to upload the data. The problem with that is that the
first glTexImage2D was always setting the internal format to
GL_BGRA_EXT and then if a 16-bit texture is used we would later call
glTexSubImage2D with a data format of GL_RGBA. Under GLES2 the
internal format must always match the data format so this is
technically invalid.

This patch makes it so that it always calls glTexImage2D when flushing
the damage for the first time. That way it will use the right internal
format and we don't need to call glTexImage2D with NULL data.

https://bugs.freedesktop.org/show_bug.cgi?id=75251