[rlgl] Compile issue with GL_HALF_FLOAT_OES on GRAPHICS_API_OPENGL_21

[ghost]

• I tested it on latest raylib version from master branch
• I checked there is no similar issue already reported
• I checked the documentation on the wiki
• My code has no errors or misuse of raylib

Issue description

While trying to compile the lastest master branch (b82217e) for PLATFORM_DESKTOP with GRAPHICS_API_OPENGL_21 on Linux I get this error:

~/test$ unzip raylib-master.zip
[...]
~/test$ cd raylib-master/src/
~/test/raylib-master/src$ make PLATFORM=PLATFORM_DESKTOP GRAPHICS=GRAPHICS_API_OPENGL_21
gcc -c rcore.c -Wall -D_GNU_SOURCE -DPLATFORM_DESKTOP -DGRAPHICS_API_OPENGL_21 -Wno-missing-braces -Werror=pointer-arith -fno-strict-aliasing  -std=c99 -fPIC -O1 -Werror=implicit-function-declaration -I. -Iexternal/glfw/include -Iexternal/glfw/deps/mingw
In file included from rcore.c:121:
rlgl.h: In function ‘rlGetGlTextureFormats’:
rlgl.h:3270:151: error: ‘GL_HALF_FLOAT_OES’ undeclared (first use in this function); did you mean ‘GL_HALF_FLOAT_ARB’?
 3270 |  RL_PIXELFORMAT_UNCOMPRESSED_R16: if (RLGL.ExtSupported.texFloat16) *glInternalFormat = GL_LUMINANCE; *glFormat = GL_LUMINANCE; *glType = GL_HALF_FLOAT_OES; break;   // NOTE: Requires extension OES_texture_half_float
      |                                                                                                                                           ^~~~~~~~~~~~~~~~~
      |                                                                                                                                           GL_HALF_FLOAT_ARB
rlgl.h:3270:151: note: each undeclared identifier is reported only once for each function it appears in
make: *** [Makefile:687: rcore.o] Error 1

The error doesn't happen if I use GRAPHICS_API_OPENGL_11 or GRAPHICS_API_OPENGL_33.

I believe this is related to a previous commit: dc621ca.

@Not-Nik I'm unfamiliar with GL_HALF_FLOAT_OES. If you can, could you please take a look at this?

Environment

Platform: Desktop
Operating System: Linux Mint 21.1 (x86_64)
OpenGL version: 3.1 Mesa 22.0.5
GPU: Intel HD Graphics 3000

[Not-Nik]

I'll look into it once I get home from work

[ghost]

Thank you very much!

[raysan5]

Please note that GRAPHICS_API_OPENGL_21 defines by default GRAPHICS_API_OPENGL_33 for convenience, probably the issue relies there.

[Not-Nik]

No it doesn't, we just aren't differentiating between OpenGL 2.1 and ES 2 so I'll add another ifdef branch for that

[Not-Nik]

Opened a PR. I also suggest we add a CI check for GL 2.1 builds so this doesn't happen again

[ghost]

@Not-Nik Thank you very much for the fix!