Reorganize how shader files sit in the shaders folder

manimlib/shaders/quadratic_bezier_stroke/vert.glsl

@@ -0,0 +1,39 @@
+#version 330
+
+uniform mat4 to_screen_space;
+uniform float is_fixed_in_frame;
+uniform float focal_distance;
+
+in vec3 point;
+in vec3 prev_point;
+in vec3 next_point;
+in vec3 unit_normal;
+
+in float stroke_width;
+in vec4 color;
+
+// Bezier control point
+out vec3 bp;
+out vec3 prev_bp;
+out vec3 next_bp;
+out vec3 v_global_unit_normal;
+
+out float v_stroke_width;
+out vec4 v_color;
+
+const float STROKE_WIDTH_CONVERSION = 0.01;
+
+// To my knowledge, there is no notion of #include for shaders,
+// so to share functionality between this and others, the caller
+// replaces this line with the contents of named file
+#INSERT position_point_into_frame.glsl
+
+void main(){
+    bp = position_point_into_frame(point);
+    prev_bp = position_point_into_frame(prev_point);
+    next_bp = position_point_into_frame(next_point);
+    v_global_unit_normal = normalize(to_screen_space * vec4(unit_normal, 0)).xyz;
+
+    v_stroke_width = STROKE_WIDTH_CONVERSION * stroke_width;
+    v_color = color;
+}