When timestamping textures, need to emit the event with SAVAGE_WAIT_3D

When timestamping textures, need to emit the event with SAVAGE_WAIT_3D
set. Otherwise the 3D engine may still be using a texture that is
being overwritten. In order to minimize the cost of waiting, timestamp
textures only when needed: when a texture image changes, when a
different texture is bound to a texture unit or when a texture unit is
disabled. (Used to be after flushing every command buffer.)