Add 240p, 480p, and 960p integer scale widescreen resolution support

[Copilot]

This PR adds three new widescreen resolution options that represent proper integer scales of the N64's native resolution, addressing the issue where existing widescreen modes cause 2D elements to look off due to non-integer scaling.

Changes Made

Added the following resolution options to the "Wide Resolution" configuration in libretro_core_options.h:

• 427x240 (16:9 Integer Scale) - 1x integer scale for 240p widescreen displays
• 854x480 (16:9 Integer Scale) - 2x integer scale for 480p widescreen displays
• 1707x960 (16:9 Integer Scale) - 4x integer scale for 960p widescreen displays

These resolutions maintain proper pixel scaling ratios from the N64's native resolution, ensuring that 2D elements render correctly without distortion or alignment issues that can occur with non-integer scaling factors.

Benefits

• Improved visual quality for 2D game elements on widescreen CRT and plasma displays
• Proper integer scaling maintains pixel-perfect rendering
• Clear labeling distinguishes these from standard 16:9 resolutions
• Positioned logically in the resolution list for easy selection

The new options are clearly labeled as "Integer Scale" to help users understand their purpose and distinguish them from the existing non-integer widescreen resolutions.

Fixes #572.

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