OPENGL: Add Missing Default Switch Cases

These are flagged by GCC if -Wswitch-default is enabled.

backends/graphics/opengl/context.cpp

@@ -170,6 +170,10 @@ void OpenGLGraphicsManager::initializeGLContext() {
 	case kContextGLES2:
 		debug(5, "OpenGL: GLES2 context initialized");
 		break;
+
+	default:
+		warning("OpenGL: Unknown context initialized");
+		break;
 	}
 
 	// Log features supported by GL context.

backends/graphics/opengl/debug.cpp

@@ -45,6 +45,8 @@ Common::String getGLErrStr(GLenum error) {
 		return "GL_STACK_UNDERFLOW";
 	case GL_OUT_OF_MEMORY:
 		return "GL_OUT_OF_MEMORY";
+	default:
+		break;
 	}
 
 	return Common::String::format("(Unknown GL error code 0x%X)", error);

backends/graphics/opengl/framebuffer.cpp

@@ -117,6 +117,8 @@ void Framebuffer::applyBlendState() {
 			GL_CALL(glEnable(GL_BLEND));
 			GL_CALL(glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA));
 			break;
+		default:
+			break;
 	}
 }