Initialize TexenvCombineState to proper defaults.

Without correct defaults some combinations miss texture data.
For example the following sequence of calls will fail to produce
RGB colors upon rendering:

  glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE)
  glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_REPLACE)
  glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_PRIMARY_COLOR)

The spec tells that correct defaults are:
  mode=GL_MODULATE
  src0=GL_TEXTURE
  src1=GL_PREVIOUS
  src2=GL_CONSTANT

See pages 185 and 279 at http://www.opengl.org/documentation/specs/version2.0/glspec20.pdf

Also updates paths in README.