LibGL+LibGPU+LibSoftGPU: Implement blend equations

This implements support for `glBlendEquation` and
`glBlendEquationSeparate`. These functions modify the calculation of the
resulting color in blending mode.

Userland/Libraries/LibGPU/Enums.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Stephan Unverwerth <s.unverwerth@serenityos.org>
+ * Copyright (c) 2024, Jelle Raaijmakers <jelle@gmta.nl>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -21,6 +22,14 @@ enum class AlphaTestFunction {
     Greater,
 };
 
+enum class BlendEquation {
+    Add,
+    Subtract,
+    ReverseSubtract,
+    Min,
+    Max,
+};
+
 enum class BlendFactor {
     Zero,
     One,