Fix error type for unavailable pixel format error

All backends reported this as GLFW_PLATFORM_ERROR, while the
documentation mandates GLFW_FORMAT_UNAVAILABLE.

src/egl_context.c

@@ -306,7 +306,7 @@ int _glfwCreateContext(_GLFWwindow* window,
 
     if (!chooseFBConfigs(ctxconfig, fbconfig, &config))
     {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
+        _glfwInputError(GLFW_FORMAT_UNAVAILABLE,
                         "EGL: Failed to find a suitable EGLConfig");
         return GL_FALSE;
     }

src/glx_context.c

@@ -321,7 +321,7 @@ int _glfwCreateContext(_GLFWwindow* window,
 
     if (!chooseFBConfig(fbconfig, &native))
     {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
+        _glfwInputError(GLFW_FORMAT_UNAVAILABLE,
                         "GLX: Failed to find a suitable GLXFBConfig");
         return GL_FALSE;
     }

src/nsgl_context.m

@@ -211,8 +211,8 @@ int _glfwCreateContext(_GLFWwindow* window,
         [[NSOpenGLPixelFormat alloc] initWithAttributes:attributes];
     if (window->nsgl.pixelFormat == nil)
     {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
-                        "NSGL: Failed to create OpenGL pixel format");
+        _glfwInputError(GLFW_FORMAT_UNAVAILABLE,
+                        "NSGL: Failed to find a suitable pixel format");
         return GL_FALSE;
     }
 

src/wgl_context.c

@@ -250,7 +250,7 @@ static GLboolean choosePixelFormat(_GLFWwindow* window,
     closest = _glfwChooseFBConfig(desired, usableConfigs, usableCount);
     if (!closest)
     {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
+        _glfwInputError(GLFW_FORMAT_UNAVAILABLE,
                         "WGL: Failed to find a suitable pixel format");
 
         free(usableConfigs);