fix [2315900] Monochrome camera preview screen after launching camera

this was introduced by a recent change. when we try to figure out the size of
the yuv->rgb temporary buffer, the output resolution has not been computed yet
and an invalid buffer size is used. most of the time the allocation fails
and the system reverts to "standard" GL will uses onle the Y plane.

the allocation of the temporary buffer is moved to onDraw(), the first
time it is called, by that time, the window is positioned properly.