Partially restore sync load and load wallpaper earlier to fix texture load time regression

[Ryunam]

Description

This is a proposed fix to address the slowdown regression introduced with some of the recent changes, namely those that added async texture loading, while still partly retaining the asynchronous process.
As observed in #18943, in certain scenarios both menu texture icons and custom wallpapers (if any) would exhibit slower load times at startup in RA, resulting in laggier menu composition.

This change attempts to fix this regression by implementing the following:

• crucial startup-related textures have been moved back to synchronous loading for Ozone, MaterialUI and XMB;
• furthermore, in XMB and MaterialUI the wallpaper loading functions have been moved earlier on in the sequence;
• an added mitigation was implemented on XMB so that, upon context reset (for example, when toggling between Fullscreen and Windowed modes), the wallpaper is loaded synchronously. This should completely eliminate situations where you might still see a brief flash of the base theme color when toggling modes, due to the wallpaper async loading taking too long to happen on certain slow-loading systems.

Related Issues

Hopefully fixes #18943

Reviewers

@LibretroAdmin

[LibretroAdmin]

crucial startup-related textures have been moved back to synchronous loading for Ozone, MaterialUI and XMB

can you list these textures?

[Ryunam]

Basically most of the menu textures, which were the ones affected by the slowdown, as defined by:

• xmb->textures.list for XMB;
• ui->textures.list for MaterialUI;
• ozone->textures, theme->textures and ozone->icons_textures for Ozone.

All playlist-related icons, on the other hand, remain asynchronous.

[LibretroAdmin]

Kinda disappointing to have to roll all that back but I'm sure you have your reasons for doing so