apps: fix integer-overflow and OOB write issues in image loaders

[kraszkow]

This patch hardens ImageBuffer and the PFM loader against malformed or malicious inputs:

• Switch width/height/channels from int to size_t to avoid signed overflow.
• Add maxDim guard (64K) and reject images whose total pixel count exceeds int limits to prevent integer multiplication overflow.
• Validate pixel reads during PFM parsing and fail early on read errors.
• Use overflow-safe widthheightC checks before allocating the buffer.
• Add static_assert ensuring size_t is 64-bit on supported platforms.

These changes prevent tiny allocations followed by large writes caused by untrusted PFM headers (e.g., malicious W/H/C), removing the possibility of heap-based out-of-bounds writes.

[Copilot]

Pull request overview

This PR hardens image loading code against malicious or malformed inputs by addressing integer overflow and out-of-bounds write vulnerabilities. The changes focus on preventing heap-based exploits that could occur when processing untrusted image files, particularly PFM format files.

Key changes:

• Switches dimension and channel types from int to size_t to prevent signed integer overflow
• Adds validation to reject excessively large images (maxDim=64K) and ensures total pixel counts don't exceed int limits
• Improves PFM loader error handling with early validation of pixel read operations

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
common/platform.h	Adds compile-time assertion ensuring size_t is 64-bit on all supported platforms
apps/utils/image_io.cpp	Changes PFM loader to use size_t for dimensions/channels and adds per-pixel read validation
apps/utils/image_buffer.h	Updates ImageBuffer interface to use size_t for dimensions and adds maxDim constant
apps/utils/image_buffer.cpp	Implements validation checks to prevent overflow during buffer allocation

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[Copilot]

@kraszkow I've opened a new pull request, #281, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

@kraszkow I've opened a new pull request, #282, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

@kraszkow I've opened a new pull request, #283, to work on those changes. Once the pull request is ready, I'll request review from you.

[atafra]

Looks good! Just a few more minor things:

• Please update commit message to: "apps: fix integer-overflow..."
• Please update changelog, without "apps:" prefix but mentioning that only oidnDenoise is affected.

[atafra]

Ready to merge!