This is needed for GLES 2 support. GLES 2 doesn't support
GL_UNPACK_ROW_LENGTH, and we shouldn't even use this constant, since a
GLES implementation could raise an error.
So set it only if neccessary, and leave it in the default state
otherwise. This also smuggles in a ES 2 fallback for glUploadTex(),
and querying an extension relevant for ES 2. For the alignment state
(GL_[UN]PACK_ALIGNMENT) do the same for symmetry. All 4 states
(alignment/rows x pack/unpack) are now assumed to be in their initial
states by default.
Also redo the PixelStorei handling in the function table. I could rebase
this, but look at the commit time.