mesa: implement TexImage*Multisample

mesa: implement TexImage*Multisample

V2: - fix formatting issues
    - generate GL_OUT_OF_MEMORY if teximage cannot be allocated
    - fix for state moving from texobj to image

V3: - remove ridiculous stencil hack
    - alter format check to not allow a base format of STENCIL_INDEX
    - allow width/height/depth to be zero, to deallocate the texture
    - dont forget to call _mesa_update_fbo_texture

V4: - fix indentation
    - don't throw errors on proxy texture targets

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>