[rcore][web] Avoid glfwGetProcAddress() to reduce code size

[vdemcak]

On Emscripten, referencing glfwGetProcAddress() or emscripten_webgl_get_proc_address() links a lookup table of every GL function into both the JS and the wasm, but rlgl only needs 6 extension functions on WebGL. Emscripten links GL functions statically, so a small loader now returns them directly. See #3713.

Applied to both rcore_web.c and the experimental rcore_web_emscripten.c. The loader body is guarded by GRAPHICS_API_OPENGL_ES2, because software rendering builds don't declare the GLES extension functions.

core_basic_window, JS + wasm gzipped: 91.4 -> 83.9 KB (WebGL 1), 94.4 -> 81.3 KB (WebGL 2).

On web, rlGetProcAddress() now only returns those 6 functions.

Tested shaders_mesh_instancing on WebGL 1 & 2: works identically.

[vdemcak]

Also, optional follow-up: -s GL_ENABLE_GET_PROC_ADDRESS=1 from projects/CMake/CMakeLists.txt should no longer be needed and can be removed.

[raysan5]

rlgl uses the loader for the following potential functions:

glGenVertexArraysOES
glBindVertexArrayOES
glDeleteVertexArraysOES

glDrawArraysInstancedANGLE
glDrawElementsInstancedANGLE
glVertexAttribDivisorANGLE

glDrawArraysInstancedEXT
glDrawElementsInstancedEXT
glVertexAttribDivisorEXT
glDrawArraysInstancedNV
glDrawElementsInstancedNV
glVertexAttribDivisorNV

Maybe the additional EXT/NV names should be also checked instead of relying only on ANGLE extension names.

[vdemcak]

On web, the only instancing extension WebGL defines is ANGLE_instanced_arrays (there's no EXT/NV variant in the WebGL extension registry), so rlgl will only ever request the *ANGLE functions there. The EXT/NV branches can't be hit on this platform.

That said, I'm happy to add the EXT/NV names too if you'd prefer it to be defensive.

[raysan5]

@vdemcak that's a nice improvement! Added some review. Also, please, could you update CMakeLists.txt?