[Omega] 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 21.0.1 and update changelog.

I'll create a backport if accepted.

[phunkyfish]

Looks good to me. Thanks for the contribution

[phunkyfish]

Question, why not define HAVE_GLES2 correctly in FindOpenGLES.cmake?

[mglae]

Question, why not define HAVE_GLES2 correctly in FindOpenGLES.cmake?

Good question. As HAVE_GLES can only be 2 or 3 it is already the correctly defined HAVE_GLES2.

PR is updated.