Fix Emscripten immediate mode GL and add comprehensive tests

- Fix immediate mode (glBegin/glEnd) by using color-per-vertex pattern
- Document Emscripten's requirement for color per vertex in GL_README.md
- Add OpenGL test tab with visual rendering verification
- Test GL_TRIANGLES, GL_QUADS, GL_LINES, GL_LINE_STRIP, GL_LINE_LOOP
- Update wxwidgets submodule with improved context management

Key finding: Emscripten's LEGACY_GL_EMULATION requires glColor*() to be
called before EACH glVertex*() call, not using OpenGL's "current color"
semantic. This is because the stride calculation expects interleaved
vertex data with color per vertex.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/GL_README.md

@@ -0,0 +1,110 @@
+# Emscripten LEGACY_GL_EMULATION - Immediate Mode Notes
+
+## Overview
+
+This document describes the behavior and limitations of Emscripten's `LEGACY_GL_EMULATION` when using OpenGL immediate mode (glBegin/glEnd) in WebAssembly builds.
+
+## Key Finding: Color Per Vertex Requirement
+
+**Emscripten's immediate mode requires color to be specified per-vertex, not using OpenGL's "current color" semantic.**
+
+### Standard OpenGL Behavior
+
+In desktop OpenGL, you can set a color once and it applies to all subsequent vertices:
+
+```cpp
+glBegin(GL_TRIANGLES);
+    glColor3f(1.0f, 0.0f, 0.0f);  // Set current color to red
+    glVertex3f(0, 0, 0);           // Uses red
+    glVertex3f(1, 0, 0);           // Still uses red
+    glVertex3f(0, 1, 0);           // Still uses red
+glEnd();
+```
+
+### Emscripten Behavior
+
+In Emscripten's LEGACY_GL_EMULATION, you **must** call `glColor*()` before **each** `glVertex*()`:
+
+```cpp
+glBegin(GL_TRIANGLES);
+    glColor3f(1.0f, 0.0f, 0.0f);  // Color for vertex 1
+    glVertex3f(0, 0, 0);
+    glColor3f(1.0f, 0.0f, 0.0f);  // Color for vertex 2 - REQUIRED!
+    glVertex3f(1, 0, 0);
+    glColor3f(1.0f, 0.0f, 0.0f);  // Color for vertex 3 - REQUIRED!
+    glVertex3f(0, 1, 0);
+glEnd();
+```
+
+### Why This Happens
+
+Emscripten's GLImmediate module calculates vertex stride based on enabled attributes:
+- Position (glVertex3f): 16 bytes (4 floats: x, y, z, w)
+- Color (glColor3f/4f): 4 bytes (4 unsigned bytes: r, g, b, a)
+- **Total stride: 20 bytes per vertex**
+
+The `numVertices` calculation is:
+```javascript
+numVertices = 4 * vertexCounter / stride
+```
+
+If color is only specified once but vertices are specified multiple times, `vertexCounter` won't be evenly divisible by `stride`, causing the assertion:
+```
+Assertion failed: `numVertices` must be an integer.
+```
+
+## Verified Working Functions
+
+The following immediate mode primitives work correctly with the color-per-vertex pattern:
+
+| Primitive | Status | Notes |
+|-----------|--------|-------|
+| GL_TRIANGLES | Working | RGB color interpolation works |
+| GL_QUADS | Working | Use glVertex3f (glVertex2f also works) |
+| GL_LINES | Working | |
+| GL_LINE_STRIP | Working | |
+| GL_LINE_LOOP | Working | |
+
+## Unsupported Functions
+
+- `glVertex2d` - Not implemented in Emscripten, use `glVertex2f` or `glVertex3f` instead
+
+## Build Flags
+
+Enable legacy GL emulation with these Emscripten flags:
+```
+-sLEGACY_GL_EMULATION
+-sMAX_WEBGL_VERSION=2
+```
+
+You may see these warnings (they are expected):
+```
+WARNING: using emscripten GL emulation. This is a collection of limited workarounds, do not expect it to work.
+WARNING: using emscripten GL immediate mode emulation. This is very limited in what it supports
+```
+
+## Implications for KiCad
+
+KiCad uses immediate mode in several places:
+1. Cursor rendering in `opengl_gal.cpp`
+2. Bitmap quad rendering
+3. Antialiasing overlays
+
+Any code that sets a color once and draws multiple vertices will need modification for WASM builds.
+
+### Possible Solutions
+
+1. **Compatibility layer**: Wrap glColor/glVertex calls to automatically replicate colors
+2. **Code modification**: Update KiCad's GAL to always specify color per-vertex
+3. **VBO migration**: Convert immediate mode code to use Vertex Buffer Objects
+
+## Test Results
+
+All immediate mode tests pass with the color-per-vertex pattern:
+- RGB triangle with smooth color interpolation
+- Yellow quad
+- White line
+- Cyan line strip
+- Magenta line loop
+
+See `wasm-app/minimal_test.cpp` for working examples.