Improve desktop pad input mapping

[GTTeancum]

Summary

• select the first available controller in slots 0 through 3 instead of hardwiring slot 0
• keep keyboard input available when a controller is connected
• map both analog sticks, including WASD and IJKL keyboard fallbacks
• convert host axes across the full PS2 0..255 range with an exact 128 center
• keep the direct host backend and libpad fallback mappings consistent

Tests

• MINITEST_FILTER=PadInput ps2x_tests.exe: 12 passed, 0 failed

The focused suite verifies the PS2-side pad packet contract. This is a draft pending live movement and camera validation in X-Men Legends.