Silence MuPDF warnings/errors unconditionally (fixes #132)

[tloriato]

Unconditionally silences MuPDF warning and error callbacks on every new context, suppressing stderr noise like warning: ignoring CMap range .... Errors are still surfaced via Go error returns.

Closes #132.

Changes

• cgo (fitz_cgo.go): static no-op callback + go_fitz_silence(ctx) called in New and NewFromMemory.
• nocgo (fitz_nocgo.go): single purego.NewCallback allocated in init(), reused for every context via applySilence.
• All 35 tests pass, go build ./..., go vet ./... clean.

[gen2brain]

Sorry, do not have much time to spend on this. At first glance, why would a user be involved at all? Warnings should be silenced; who wants them in the output? Errors should be returned as errors. How useful would be for someone to log and collect warnings. As for purego and NewCallback, just a note that there is a limited number of callbacks one can create so it should be reused if possible.

[tloriato]

Hey @gen2brain! Thank you for looking, I tried to change and make as brief as possible so it doesn't take your time. What I reworked:

• Dropped the Quiet flag — warnings and errors are now silenced unconditionally on every new context. No user involvement.
• Errors continue to surface through Go error returns (the existing fz_try/fz_catch wrappers already handle that).
• The purego callback is allocated once in init() and reused.
• Removed the SetWarningFunc idea entirely.

Net result is -35 / +9 lines vs the previous revision. All 35 tests pass.

[gen2brain]

Warnings are silenced in d009362 . Closing this one.