(XMB/GLUI/Ozone) Add optional menu screensaver animations

[jdgleaver]

Description

This is a follow-up to #12217.

At present, when using XMB/GLUI/Ozone, enabling the menu screensaver will cause the screen to be blanked when the screensaver timeout has elapsed. This PR adds three optional animations that may be displayed instead - these can be selected via the new Settings > User Interface > Menu Screensaver Animation entry; animation speed may be adjusted via Settings > User Interface > Menu Screensaver Animation Speed. The options are as follows:

• OFF: Screen will be blanked

• Snow:

• Star Field:

• Vortex:

Notes:

• These effects are rendered using font glyphs; the performance overheads of all the animations are therefore low
• The animations are dependent upon the font file assets_dir/pkg/osd-font.ttf. Most platforms should have this installed by default. If the font file is missing (or on platforms without TTF font support), the animations will still run, but all glyphs will be replaced by * from RA's internal bitmap font
• When using GLUI/Ozone, the screensaver will be tinted (somewhat) according to the current menu colour theme (XMB uses a fixed screensaver colour, since it does not have a 'proper' colour theme implementation, and adding that is beyond the scope of this PR...)