fixed compiler warnings reported by gcc/clang

* changed some catch blocks to use exception references instead of value types
 * fixed missing use of SCR_WIDTH/SCR_HEIGHT for glfwCreateWindow
 * fixed implicit numeric type casts regarding signedness
 * removed unused variables

Signed-off-by: Toni Uhlig <matzeton@googlemail.com>

includes/learnopengl/shader.h

@@ -53,7 +53,7 @@ public:
                 geometryCode = gShaderStream.str();
             }
         }
-        catch (std::ifstream::failure e)
+        catch (std::ifstream::failure& e)
         {
             std::cout << "ERROR::SHADER::FILE_NOT_SUCCESFULLY_READ" << std::endl;
         }
@@ -188,4 +188,4 @@ private:
         }
     }
 };
-#endif
+#endif