Fix sRGB exponent in Metal rendering (#2846)

This tiny change should 'fix' the sRGB encoding used on the metal backend.

I noticed this while using the MaterialXViewer in a CI setup when generating images where no single pixel matched against reference values (not even the background color :D)

Author: exavi

source/MaterialXRenderMsl/MetalState.mm

@@ -82,7 +82,7 @@
                  "    for(int i = 0; i < 3; ++i)                                                  \n"
                  "        color_srgb[i] = (color_linear[i] < 0.0031308) ?                         \n"
                  "            (12.92 * color_linear[i])                 :                         \n"
-                 "            (1.055 * pow(color_linear[i], 1.0h / 2.2h) - 0.055);                \n"
+                 "            (1.055 * pow(color_linear[i], 1.0h / 2.4h) - 0.055);                \n"
                  "    color_srgb[3] = color_linear[3];                                            \n"
                  "    return color_srgb;                                                          \n"
                  "}                                                                               \n"
@@ -146,7 +146,7 @@
              "    for(int i = 0; i < 3; ++i)                                \n"
              "        color_srgb[i] = (color_linear[i] < 0.0031308) ?       \n"
              "          (12.92 * color_linear[i])                   :       \n"
-             "          (1.055 * pow(color_linear[i], 1.0h / 2.2h) - 0.055);\n"
+             "          (1.055 * pow(color_linear[i], 1.0h / 2.4h) - 0.055);\n"
              "    color_srgb[3] = color_linear[3];                          \n"
              "    return color_srgb;                                        \n"
              "}                                                             \n"