Fix color leakage issue in shader rendering

[mathsvisualization]

Fix Color Leakage in Fragment Shader

Motivation

While rendering overlapping triangles with opposite orientations,
minor color leakage artifacts were observed.

The issue was caused by aggressive alpha scaling in the fragment shader:

float a = 0.95 * frag_color.a;

This resulted in excessive blending, especially when negatively oriented triangles were involved.

Proposed Changes

• Alpha scaling factor was reduced from 0.95 to 0.90:

float a = 0.90 * frag_color.a;

This lowers the blending strength and prevents unintended color bleed.

• Orientation-Based Alpha Correction
  For negatively oriented triangles, the following transformation is used:

if (orientation < 0)
    a = -a / (1 - a);

Mathematically, the modified alpha value is:
a' = -a / (1 - a)
This ensures correct cancellation behavior under the standard blending equation:
(1 - alpha) * dst + alpha * src
The transformation guarantees that blending a positively oriented triangle followed by a negatively oriented one restores the original fragment color.

Testing

• Render overlapping triangles with opposite orientations
• Verify no residual tint remains
• Confirm stable blending behavior

from manimlib import *

class GlyphTest1(InteractiveScene):
    def construct(self):
        char = "B"
        t = TexText(char)
        t.set_height(FRAME_HEIGHT - 2)
        self.add(t)

Result

• Cleaner blending
• No color leakage
• Stable fragment output

Before

After