Summary

This PR improves IME integration in GLFW for desktop text input workflows.

What’s included

1. New API: glfwSetInputMethodCursorPos(GLFWwindow* window, double xpos, double ypos)
   • Lets applications provide the current text cursor anchor position (content-area coordinates).
   • ntended for IME candidate/composition UI positioning.
2. macOS (Cocoa) improvements
   • firstRectForCharacterRange now uses the provided input method cursor position and converts it to screen coordinates.
   • During IME composition, editing keys (Backspace/Delete/Enter/Arrow/Home/End/Escape) are not forwarded to app key handlers, preventing accidental deletion of already committed text.
   • Composition state is tracked from setMarkedText / unmarkText / insertText.
3. Win32 improvements
   • glfwSetInputMethodCursorPos updates IME candidate/composition windows via:
     • ImmSetCandidateWindow
     • ImmSetCompositionWindow
   • Position is converted with window content scale so placement remains correct under DPI scaling.
4. Build update
   • Links imm32 when building Win32 backend.

Motivation

In real-world IME usage:

• Candidate windows should appear near the text caret, not at a fallback corner.
• During preedit/composition, Backspace should edit the preedit text only, not delete committed text in the application input field.
  These issues are especially visible with CJK input methods.

API Notes

• The new function stores an IME anchor position per window.
• It is safe to call every frame / whenever caret position changes.
• Non-implemented platforms will still compile and keep existing behavior.

Files changed

• glfw3.h
• internal.h
• input.c
• cocoa_window.m
• CMakeLists.txt

Validation

Tested locally with:

• macOS: candidate window follows caret; Backspace during composition no longer deletes committed app text.
• Win32 code path implemented and build linkage updated (imm32).