[Nexus] Fix Admin window on GLES / Warn if skin is not supported

[mglae]

Fix for #141 "Client Specific Settings" were only shown with GL.

HAVE_GLES2 was never defined in FindOpenGLES.cmake. In addition vert.glsl needed to be fixed:

2023-02-27 15:52:04.925 T:1734     info <general>: Loading skin file: /storage/.kodi/addons/pvr.vdr.vnsi/resources/skins/skin.estuary/xml/Admin.xml, load type: LOAD_ON_GUI_INIT
2023-02-27 15:52:04.931 T:1734    debug <general>: [Warning] CGUITextureManager::GetTexturePath: could not find texture '-'
2023-02-27 15:52:04.932 T:1734    error <general>: AddOnLog: pvr.vdr.vnsi: CVertexShader::Compile: 0:10(13): error: `m_proj' undeclared
                                                   0:10(22): error: `m_model' undeclared
                                                   0:10(13): error: operands to arithmetic operators must be numeric
                                                   
2023-02-27 15:52:04.932 T:1734    error <general>: AddOnLog: pvr.vdr.vnsi: GL: Error compiling vertex shader

With this PR the Admin window is visible in LibreELEC Generic (GBM).

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Second commit is to add a warning dialog if the skin is not supported.

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Increase version to 20.4.1 and update changelog.

Backport of #203

[gade01]

@mglae @phunkyfish
I'm having some errors at Weblate from this PR.
It seems string id 30205

msgctxt "#30205"
msgid "Repeating Child"
msgstr "" 

is an exact duplicate of the string above.
Is this on purpose?

https://github.com/kodi-pvr/pvr.vdr.vnsi/pull/204/files#diff-52bcc2968fb526b23369a05af0c6a002b9edab9a5c198eabbc6ae39d72015272R323-R325

[phunkyfish]

Definitely a mistake. @mglae can you PR the change to fix? Please also check the omega branch but I assume it’s the same.

[mglae]

@gade01 @phunkyfish PRs are created.

[phunkyfish]

Thanks, will merge shortly.

[gade01]

Thanks