driver: implement TEXP

Projective texturing instruction, mostly generated by GLES1
compatibility shaders.
ps_sandbox: use TRIANGLE_FAN

This primitive had gone untested until now.
Just like the Gallium blitter use a TRIANGLE_FAN to render a quad.
Also cleans up the code of this demo as no longer an index buffer is
needed.
driver: don't zero-initialize resources

Nothing is guaranteed about uninitialized resource contents,
so don't waste CPU cycles on this.