treewide: make a hardware path for util_clear_texture and remove PIPE_CAP_CLEAR_TEXTURE
Before util_clear_texture implemented clear_texture through a memset. This patch makes util_clear_texture use either pipe->clear_texture or fallback to util_clear_texture_common, which implements clear texture in HW with a software fallback for incompatible formats.
Then we make both state_tracker and rusticl use the new helper, making the ARB_clear_texture extension available for every driver.