Use the WebGL 1.0 specific constant for the half float format.

[aglitchman]

WebGL 1.0 supports loading half float textures. But it requires the use of HALF_FLOAT_OES constant, which has a different value from the HALF_FLOAT constant from WebGL 2.0 / OpenGL ES 3.0. This fix enables to use half float texture format on WebGL 1.0 devices.

Fixes #8241

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[aglitchman]

In fact and from my view, for WebGL we should use header files from ES 2.0/3.0, because it is based on it. But someone decided to use files from desktop GL. That's strange. And there is no GL_HALF_FLOAT_OES in desktop GL. So I added this constant manually as #define.

PS If the desktop include above is replaced by include gl2ext.h, for example, the webgl renderer stops working and pulls a huge amount of patching and testing.

[Jhonnyg]

I think you should put this into graphics_opengl_defines.h and follow the style of the other defines

[aglitchman]

Okay 🙌🏻 Done.

[aglitchman]

I made a build of the engine myself and tested half float texture on WebGL 1.0 device - works great.

The only thing for WebGL 1.0 is that half float texture must be POT.

[Jhonnyg]

If this requires POT textures, we should probably output an error if someone is trying to use a wrong combination, otherwise we might just get a black screen and no info on why it is happening

[Jhonnyg]

I think you should put this into graphics_opengl_defines.h and follow the style of the other defines

[aglitchman]

If this requires POT textures, we should probably output an error if someone is trying to use a wrong combination, otherwise we might just get a black screen and no info on why it is happening

Ok 🙌🏻 I think it should be a new issue and a new PR (like "The engine should warn that the texture should be POT").

[aglitchman]

Looks to me like everything is ok now and you can do the merge.