gl/glext: More general extension query support

+ Enter extensions into a vector so they can be queried faster without
  having to add a new function per extension
gl/texture: Texture views (untested)

+ Create a texture view from a different texture; untested, because this
  doesn't work for depth/stencil textures (thanks based ARB)
gl/framebuffer: Stencil buffer support (untested)

+ Untested, because using both a stencil and a depth attachment do not
  work; creating a texture view from a combined stencil/depth buffer
  does not work either; and using both the stencil and the depth part at
  the same time is impossible as well (thanks based ARB)

“there's no great need” — ARB_texture_view