vc4: Add support for rebasing texture levels so firstlevel == 0.

GLES2 doesn't have GL_TEXTURE_BASE_LEVEL, so the hardware doesn't.  Fixes
piglit levelclamp, tex-miplevel-selection, and texture-storage/2D mipmap
rendering.
vc4: Translate 4-byte index buffers to 2 bytes.

Fixes assertion failures in 14 piglit tests (half of which now pass).