Userland: static vs non-static constexpr variables

Problem:
- `static` variables consume memory and sometimes are less
  optimizable.
- `static const` variables can be `constexpr`, usually.
- `static` function-local variables require an initialization check
  every time the function is run.

Solution:
- If a global `static` variable is only used in a single function then
  move it into the function and make it non-`static` and `constexpr`.
- Make all global `static` variables `constexpr` instead of `const`.
- Change function-local `static const[expr]` variables to be just
  `constexpr`.

Userland/Libraries/LibGfx/Color.h

@@ -140,7 +140,7 @@ public:
 #endif
     }
 
-    constexpr Color interpolate(const Color& other, float weight) const
+    Color interpolate(const Color& other, float weight) const
     {
         u8 r = red() + roundf(static_cast<float>(other.red() - red()) * weight);
         u8 g = green() + roundf(static_cast<float>(other.green() - green()) * weight);