Fix VDP2 background rendering pipeline

[spacestate1]

Fixes VDP2 background layer rendering so layers display correctly at all resolutions.

Files changed

VDP2 shader

• AzelLib/shaders/VDP2_ps.sc (L6): Use regular sampler for plane config texture (was unsigned int sampler that broke on some drivers)
• AzelLib/shaders/VDP2_ps.sc (L42-48): Read config values as RGBA8 bytes and reconstruct ints (matches new texture format)
• AzelLib/shaders/VDP2_ps.sc (L120-127): Wrap scroll coordinates with modulo so negative scroll values work instead of showing red
• AzelLib/shaders/VDP2_ps.sc (L131): Fix Y-axis tile lookup - was using width instead of height
• AzelLib/shaders/VDP2_ps.sc (L163-185): Pick correct palette based on color depth (16-color vs 256-color modes)
• AzelLib/shaders/VDP2_ps.sc (L295-299): Apply zoom/scale to pixel coordinates

Layer data and GPU rendering

• AzelLib/renderer.cpp (L540-564): Add zoom and output height fields to layer config struct
• AzelLib/renderer.cpp (L582-590): Recreate GPU framebuffer when switching between GPU and software mode
• AzelLib/renderer.cpp (L617): Change config texture format from R32U to RGBA8 (works on all backends)
• AzelLib/renderer.cpp (L632-697): Pass all layer settings as GPU uniforms so the shader can read them reliably

Software rendering path

• AzelLib/renderer.cpp (new, ~L743-775): New renderLayerSoftware() function - renders on CPU and uploads to GPU texture. Replaces assert(0) that crashed when software mode was selected.
• AzelLib/renderer.cpp (L813-816): Apply zoom in software renderer too

Per-layer fixes (BG0, BG1, BG3, RBG0)

• AzelLib/renderer.cpp renderBG0 (L988-1004): Remove duplicate resolution doubling, add zoom registers, use new software path
• AzelLib/renderer.cpp renderBG1 (L1104-1163): Add zoom registers, use new software path
• AzelLib/renderer.cpp renderBG3 (L1226-1240): Add output height, use new software path
• AzelLib/renderer.cpp renderRBG0 (L1307-1323): Add output height, use new software path

Compositing (final image assembly)

• AzelLib/renderer.cpp (L1808-1812): Enable alpha blending when drawing layers, stop clearing between each layer
• AzelLib/renderer.cpp (L1854-1870): Handle all 8 HRESO resolution modes. Split VDP1 (3D) resolution from VDP2 (background) resolution - title screen needs 352px for 3D but 704px for backgrounds
• AzelLib/renderer.cpp (L1998-2040): Use the back screen color register for clear color. Draw sprites at their priority level, then backgrounds in order

Supporting fixes

• AzelLib/renderer.h (L15-17): Reorder views so VDP1 renders before the composite pass
• AzelLib/VDP2.cpp (L418-419): Init zoom Y registers to 1.0 (was uninitialized)
• AzelLib/VDP2.cpp (L1759): Fix missing return in computeStringLength()

Merge note

Safe to merge on its own. This is the biggest PR but all changes are self-consistent - the shader, texture format, and renderer code all match each other. Recommend merging this before PR 4 (title screen text), since the text layer needs these rendering fixes to actually show up.