Fix some textures not being sent correctly to older clients

Fix some textures not being sent correctly to older clients

Since b2eb44afc50976dc0954c868977b5829f3ff8a19, a texture defined as
`[combine:16x512:0,0=some_file.png;etc`
will not be sent correctly from a 5.5 server to a 5.4 client due to the
overeager detection of unsupported base modifier `[` introducing a
spurious `blank.png^` before the modifier.

Fix this by whitelisting which base modifiers can be passed through
unchanged to the client, and prefix `blank.png` for the others
(which at the moment is just [png:, but the list may grow larger
as new base modifiers are added.)