Fix file drop event cursor position reporting

Previously, file drop events always reported cursor position as (0,0)
because the position was hardcoded. GLFW's drop callback does not provide
cursor position directly, so we now query it using glfwGetCursorPos() at
the time of the drop.

This matches the approach used in GLFW's Windows backend, which updates
the cursor position immediately before calling the drop callback. On X11,
the cursor position is tracked during drag operations, so querying it at
drop time provides the accurate drop location.

Note: This is technically a separate bug fix but is included with the
GLFW 3.4 upgrade to ensure file drop events work correctly for testing
the upgraded GLFW integration.

Author: andrewfb

src/cinder/app/linux/AppImplLinuxGlfw.cpp

@@ -317,7 +317,10 @@ class GlfwCallbacks {
 			files.push_back( paths[i] );
 		}
 
-		vec2 dropPoint = { 0, 0 }; // note: doesn't appear to be any way to get the drop position.
+		// Get the cursor position at the time of the drop
+		double xpos, ypos;
+		::glfwGetCursorPos( glfwWindow, &xpos, &ypos );
+		vec2 dropPoint = { static_cast<float>(xpos), static_cast<float>(ypos) };
 		FileDropEvent dropEvent( getWindow(), dropPoint.x, dropPoint.y, files );
 		getWindow()->emitFileDrop( &dropEvent );
 	}