[video_player_avplay] Fix picture subtitle display issue

[gin7773]

Main changes:

• Fix picture subtitle display issue

[gemini-code-assist]

Code Review

This pull request enables picture subtitle display for DASH streams by removing conditional compilation flags in plus_player.cc. However, the newly exposed code contains unsafe numeric operations on untrusted input, which could lead to integer overflows, undefined behavior, and out-of-bounds reads. This poses a significant security risk, potentially causing a denial-of-service (crash) or memory corruption when processing malicious subtitle data. The package version has also been updated to 0.8.3 in pubspec.yaml, CHANGELOG.md, and README.md.

[gemini-code-assist]

The removal of #ifdef PICTURE_SUBTITLE_SUPPORT enables code that processes picture subtitles. This code contains several unsafe numeric operations on untrusted input (picture_width and picture_height from the player).

1. Undefined Behavior (UB) in channels calculation (line 1330): int channels = size / area;. If area is very small (e.g., due to underflow of picture_width * picture_height), size / area can result in Inf. Casting Inf to int is undefined behavior.
2. Integer Overflow in stride_in_bytes (line 1335): int stride_in_bytes = static_cast<int>(picture_width) * channels;. If picture_width is large (e.g., 2e9) and channels is 2, the product overflows int.
3. Undefined Behavior in static_cast<int>(picture_width) (line 1335, 1339): If picture_width exceeds INT_MAX, the cast is undefined behavior.

These issues can lead to an incorrect stride_in_bytes or dimensions being passed to stbi_write_png_to_mem (line 1337), which can cause an out-of-bounds read from the data buffer, potentially leading to a crash (DoS) or memory corruption.