fix(motionblur): stop per-frame ShaderGroup leak and edge-row artifacts

[gaoyu06]

Problem

Users report intermittent screen corruption with MotionBlur enabled: a few garbage pixel rows, or the whole top half of the screen rendering garbage.

Root cause (New mode, the default): renderOverlay reloaded both post shaders every time isUsingShader() returned false, and vanilla EntityRenderer.loadShader never deletes the previous ShaderGroup. The GLSL 150 variant (motionblur_core) mixes a #version 120 vertex shader (vanilla sobel.vsh) with a #version 150 fragment shader and cannot compile in the legacy GL context 1.8.9 gets on macOS. When it fails, useShader stays false, the name check in isUsingShader() (which only accepted the core variant's name) never matches, and the module reloads both shaders every frame — leaking two fullscreen framebuffers (color + depth) per frame until VRAM exhaustion corrupts rendering. Even where both variants load, one full ShaderGroup leaked on every activation / respawn / dimension change.

Fix

• Delete the old group (stopUseShader()) before loading, load only the GLSL 120 variant, and never retry after a failed load (log instead).
• Rebind the main framebuffer after loadShader, which leaves FB 0 bound mid-frame.
• isUsingShader() now checks for the shader that is actually loaded.
• Remove the motionblur_core assets (unreferenced now; the 120 pipeline runs everywhere 1.8.9 runs).
• Cache the listShaders reflection Field with a searge-name (field_148031_d) fallback so the Strength setting also works in the remapped production jar, and log instead of silently swallowing reflection failures.
• Old mode: fullscreen quads used integer division (width / scaleFactor), leaving up to scaleFactor-1 unprocessed pixel rows/columns at the bottom/right edge ("a few abnormal pixel rows"); now float division. Also push/pop the projection/modelview matrices around blur() instead of clobbering them.

Testing

• ./gradlew compileJava passes.
• Vanilla behavior verified against decompiled 1.8.9 (EntityRenderer.loadShader / ShaderGroup / ShaderLoader): load-shader leak, compile-failure path, and searge field name confirmed.

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