fix(build): hoist GL_GLEXT_PROTOTYPES out of __APPLE__ branch

[DTW-Thalion]

Problem

On Linux with clang 16+ (clang 18.1.3 on Ubuntu 24.04), building Source/CARenderer.m fails with:

CARenderer.m:451:11: error: call to undeclared function 'glUseProgram';
  ISO C99 and later do not support implicit function declarations

and a handful of similar errors for other GL 2.0+ entry points.

Root cause

Source/CABackingStore.h sets GL_GLEXT_PROTOTYPES only inside the __APPLE__ branch:

#if (__APPLE__)
#define GL_GLEXT_PROTOTYPES 1
#import <OpenGL/OpenGL.h>
#import <OpenGL/gl.h>
#import <OpenGL/glu.h>
#else
#import <GL/gl.h>    // no GL_GLEXT_PROTOTYPES defined here!
#import <GL/glu.h>
#endif

On Linux, <GL/gl.h> then transitively includes <GL/glext.h> — but since GL_GLEXT_PROTOTYPES isn't set at that point, glext.h sets its own include guard __gl_glext_h_ without emitting any OpenGL 2.0+ prototype declarations. CARenderer.m's later #import <GL/glext.h> with GL_GLEXT_PROTOTYPES set is then a no-op because of the guard, and glUseProgram and friends end up as implicit declarations.

Older clang (≤15) treated implicit declarations as a warning; clang 16 promoted it to an error by default, which is what finally surfaced the latent bug.

Fix

Move #define GL_GLEXT_PROTOTYPES 1 above the __APPLE__ split, so every backend has it set before the first time any glext.h is traversed. All OpenGL 2.0+ prototypes are then emitted on first entry, and the later glext.h imports in CARenderer.m continue to work.

Platform impact

• Linux/Mesa: CARenderer.m now compiles cleanly against clang 16+ and Mesa's headers.
• macOS: unchanged — the #define was already set on this path; hoisting it is a no-op.
• Windows: unchanged — current MSYS2 clang builds don't go through this include chain.

Test plan

• Ubuntu 24.04 + clang 18.1.3 + Mesa libgl1-mesa-dev 25.2: libs-quartzcore framework builds cleanly
• Verified preprocessor output: glUseProgram, glCreateShader, glLinkProgram, glGetUniformLocation etc. are all declared at the point of use
• macOS spot-check (don't have a Mac; the changed line is platform-agnostic)