Initialise keep_original_textures and use_fast_texture_filter in storage
config. Removed any other variables from storage config that were both unused
and uninitialised to avoid future confusion (if they're needed it's
easier to spot an uninitialised variable problem in a PR that adds the
variable again rather than just uses it).
Copied storage Texture struct constructor from GLES3 implementation
(except where variables were already initialised with different values).
Gives us sensible tested defaults for previously uninitialised vars.
Added assignments for state.current_main_tex based on same in GLES3.
352b3c9c93