Refactor: Avoid processing arguments directly

Avoid processing function arguments directly and instead create
temporary variables for adjustments. This is done to make it easier to
maintain identical code to CPU.

source/blender/compositor/realtime_compositor/shaders/library/gpu_shader_compositor_bright_contrast.glsl

@@ -2,10 +2,8 @@
  *
  * SPDX-License-Identifier: GPL-2.0-or-later */
 
-/* The algorithm is by Werner D. Streidt
- * (http://visca.com/ffactory/archives/5-99/msg00021.html)
- * Extracted of OpenCV demhist.c
- */
+/* The algorithm is by Werner D. Streidt, extracted of OpenCV demhist.c:
+ *   http://visca.com/ffactory/archives/5-99/msg00021.html */
 
 #include "gpu_shader_common_color_utils.glsl"
 
@@ -14,27 +12,27 @@
 void node_composite_bright_contrast(
     vec4 color, float brightness, float contrast, const float use_premultiply, out vec4 result)
 {
-  brightness /= 100.0;
+  float scaled_brightness = brightness / 100.0;
   float delta = contrast / 200.0;
 
   float multiplier, offset;
   if (contrast > 0.0) {
     multiplier = 1.0 - delta * 2.0;
     multiplier = 1.0 / max(multiplier, FLT_EPSILON);
-    offset = multiplier * (brightness - delta);
+    offset = multiplier * (scaled_brightness - delta);
   }
   else {
     delta *= -1.0;
     multiplier = max(1.0 - delta * 2.0, 0.0);
-    offset = multiplier * brightness + delta;
+    offset = multiplier * scaled_brightness + delta;
   }
 
+  vec4 input_color = color;
   if (use_premultiply != 0.0) {
-    color_alpha_unpremultiply(color, color);
+    color_alpha_unpremultiply(input_color, input_color);
   }
 
-  result.rgb = color.rgb * multiplier + offset;
-  result.a = color.a;
+  result = vec4(input_color.rgb * multiplier + offset, input_color.a);
 
   if (use_premultiply != 0.0) {
     color_alpha_premultiply(result, result);