opengl: fix orientation matrix initialization
The orientation matrix was not initialized if orientation was ORIENT_NORMAL.
The regression has been introduced by commit 01db27e1: since the matrix has been reduced to 2x3, all the matrix components were set by the macro MATRIX_SET(), so the prealable initialization to "identity" has been removed.
However, the switch did not cover the case ORIENT_NORMAL, which had nothing to do if the matrix was already initialized to "identity".
In practice, this caused a problem only on Android with hardware acceleration enabled, because interop_android.c is the only interop which does not apply a vflip to the input format (see commit 1fee4df0).