st/mesa: ignore gl_texture_object::BaseLevel when allocating gallium textures

st/mesa: ignore gl_texture_object::BaseLevel when allocating gallium textures

Previously, when we created a gallium texture for a corresponding Mesa
texture we'd only allocate space for mipmap levels >= BaseLevel.

This patch undoes that mechanism.  This fixes a render-to-texture bug
when rendering to level 0 when BaseLevel=1.

Also, it makes sense to allocate the whole texture object memory when
BaseLevel > 0 since a common use of GL_TEXTURE_BASE_LEVEL is to
progressively load/render mipmaps.  Eventually, the app almost always
fills in the level=0 mipmap image.

Finally, the texture image code is bit easier to understand now.