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.

src/mesa/state_tracker/st_atom_texture.c

@@ -138,7 +138,6 @@ finalize_textures(struct st_context *st)
          const GLuint texUnit = fprog->Base.SamplerUnits[su];
          struct gl_texture_object *texObj
             = st->ctx->Texture.Unit[texUnit]._Current;
-         struct st_texture_object *stObj = st_texture_object(texObj);
 
          if (texObj) {
             GLboolean flush, retval;
@@ -149,8 +148,6 @@ finalize_textures(struct st_context *st)
                st->missing_textures = GL_TRUE;
                continue;
             }
-
-            stObj->teximage_realloc = TRUE;
          }
       }
    }