* common: Add thread pool from yuzu
* Is really useful for asynchronous operations like shader compilation and custom textures, will be used in following PRs
* core: Improve ImageInterface
* Provide a default implementation so frontends don't have to duplicate code registering the lodepng version
* Add a dds version too which we will use in the next commit
* rasterizer_cache: Rewrite custom textures
* There's just too much to talk about here, look at the PR description for more details
* rasterizer_cache: Implement basic pack configuration file
* custom_tex_manager: Flip dumped textures
* custom_tex_manager: Optimize custom texture hashing
* If no convertions are needed then we can hash the decoded data directly removing the needed for duplicate decode
* custom_tex_manager: Implement asynchronous texture loading
* The file loading and decoding is offloaded into worker threads, while the upload itself still occurs in the main thread to avoid having to manage shared contexts
* Address review comments
* custom_tex_manager: Introduce custom material support
* video_core: Move custom textures to separate directory
* Also split the files to make the code cleaner
* gl_texture_runtime: Generate mipmaps for material
* custom_tex_manager: Prevent memory overflow when preloading
* externals: Add dds-ktx as submodule
* string_util: Return vector from SplitString
* No code benefits from passing it as an argument
* custom_textures: Use json config file
* gl_rasterizer: Only bind material for unit 0
* Address review comments
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