Black Texture on Degenerate Surface Regions

### Describe the bug
The texture does not render correctly on certain TexturedSurfaces (Sphere, Cone, Disk3D). In specific areas it appears completely black.

**Code**:
```py
from manimlib import *

class SurfaceExample(Scene):
CONFIG = {
"camera_class": ThreeDCamera,
}

def construct(self):
frame = self.camera.frame
frame.set_euler_angles(
theta=55 * DEGREES,
phi=70 * DEGREES,
)

sphere = Sphere(radius=2.5, resolution=(4, 4))

texture = "https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Whole_world_-_land_and_oceans.jpg/1280px-Whole_world_-_land_and_oceans.jpg"
earth = TexturedSurface(sphere, texture, opacity=0.9)

sphere.shift(LEFT * 3)
earth.shift(RIGHT * 3)

self.play(FadeIn(sphere))
self.play(FadeIn(earth))

self.wait()
```

**Wrong display or Error traceback**:

It is mainly noticeable in surfaces with low resolution:

Resolution (4, 4):

Resolution (11, 11):

### Additional context

The issue occurs at surface points where one of the derivative vectors is degenerate:

point − du_point = 0 or point − dv_point = 0

### Potential Fix

I added a conditional check in the texture_surface shader to detect du or dv with zero lengths, in which case the unit_normal must be calculated in a different way.

shaders/textured_surface/vert.glsl:
```glsl
#version 330

in vec3 point;
in vec3 du_point;
in vec3 dv_point;
in vec2 im_coords;
in float opacity;

out vec3 v_point;
out vec3 v_unit_normal;
out vec2 v_im_coords;
out float v_opacity;

#INSERT emit_gl_Position.glsl
#INSERT get_unit_normal.glsl

void main(){
v_point = point;
vec3 du = du_point - point;
vec3 dv = dv_point - point;

if(length(dv) < 1e6 || length(du) < 1e-6){
v_unit_normal = normalize(point);
} else {
v_unit_normal = normalize(cross(normalize(du), normalize(dv)));
}

v_im_coords = im_coords;
v_opacity = opacity;
emit_gl_Position(point);
}
```

The analytical normal of a sphere was used in the fallback case.

**Results**

Resolution (4, 4):

Resolution (11, 11):

**Conclusion**

I am not sure if this was the ideal solution, but it is working perfectly for various resolutions and surfaces now!