[tx] stdin input not fully functional

[miguelsousa]

This works

$ tx < tx_data/input/svg.svg
## Filename stdin
## Top Dict
FullName            "SourceSansPro-Regular"
sup.nGlyphs         5
## FontDict[0]
FontName            "SourceSansPro-Regular"
## Private
## glyph[tag] {name,encoding}
glyph[1] {.notdef,-}
glyph[2] {a,U+0061}
glyph[3] {gid00002,U+0020}
glyph[4] {gid00003,U+E000}
glyph[5] {gid00004,U+E001}

but this does not

$ cat tx_data/input/svg.svg | tx
tx: --- stdin
tx: (sfr) can't read source stream
tx: fatal error

[anthrotype]

I suspect it might be because somewere tx is attempting to call seek on the input stream but this is unseekable.
If it needs to seek, then it should load everything from stdin to memory and work from there.