#1078 TextBox masking in window mode.

[daneo1989]

• UpdateTextBoxHostLocation now centralizes where the WinForms input control lives. In fullscreen it sits where it always was; in windowed mode it’s moved to HiddenTextBoxLocation so it never paints over Mir UIs.
• Mouse input paths (OnMouseDown/Move/Up/DoubleClick/Wheel) detect when the control is hidden and replay the corresponding Windows messages via SendMessage, using CMain.MPoint to compute the caret position. Keyboard input still flows through the real TextBox, so all selection/editing behavior is preserved without rewriting it manually.
• Texture creation is still triggered everywhere (windowed + fullscreen), so the Mir-themed mask/caret drawn from TextBox.DrawToBitmap now matches in both modes.

@Suprcode not sure whether this is the correct direction which AI provided.