egl_sys: Drop duplicate manual definition of (EGL_)TEXTURE_FORMAT

[MarijnS95]

For #1668 (comment)

The generated code already defines TEXTURE_FORMAT to 0x3080 with the usual EGL_ prefix stripped; we shouldn't be redefining it (with this unnecessary prefix) in manual code.

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

[MarijnS95]

Right above there's a comment:

// Possible values for EGL_TEXTURE_FORMAT.

I'm carefully considering to rename that to glutin_egl_sys::TEXTURE_FORMAT. Thoughts?

[kchibisov]

I know that certain constants in spec have different names, but the same values, the same with e.g. some drm pixel formats, etc, etc. So if both names exist in spec, I won't mind keeping both of them.

[MarijnS95]

I think that there's only EGL_TEXTURE_FORMAT. As with everything that's output by gl-generator, the common EGL_ prefix is stripped off, and @chrisduerr in #1668 simply missed that it was already available in the autogenerated bindings? The value is the same too.