Fix textures not being converted to linear RGB

Previously, textures were being loaded in as SRGB and then used by the
lighting calculations as if they were in a linear RGB colour space. This
was resulting in the textures looking somewhat too bright.

Instead, textures are now converted into linear RGB with gamma 2.2 when
being loaded from their respective image.

src/texture.cpp

@@ -1,3 +1,4 @@
+#include <cmath>
 #include <sstream>
 
 #include <stb_image.h>
@@ -26,9 +27,9 @@ namespace hw4 {
                 int off = y * width + x;
 
                 data_copy[off] = glm::vec3(
-                    static_cast<float>(data[off * 3]) / 255,
-                    static_cast<float>(data[off * 3 + 1]) / 255,
-                    static_cast<float>(data[off * 3 + 2]) / 255
+                    std::pow(static_cast<float>(data[off * 3]) / 255, 2.2),
+                    std::pow(static_cast<float>(data[off * 3 + 1]) / 255, 2.2),
+                    std::pow(static_cast<float>(data[off * 3 + 2]) / 255, 2.2)
                 );
             }
         }