Normalize GL_UNPACK_ALIGNMENT

Several places were setting GL_UNPACK_ALIGNMENT
unneccessarily, whereas other places were assuming an
unpack alignment of 1. Since we never actually
do explicit row-alignment, set GL_UNPACK_ALIGNMENT
to 1 at context creation time and never change it

Bug: 26584230

Also turns on aggressive glGetError checking to
better catch potential problem zones

Change-Id: I190c8f0f0494a7f046d5ed769405c75d363be59a