Default shader version selection to GLSL300 for GLES3

[riccardobl]

JME shader version default ( GLSL100 ) selects the minimum glsl version available in the platform.

The problem is that in GLES3 it defaults to GLSL100 (GLES2) for every shader that is not specifically hinted to GLSL300, this means that GLSL>=150 && GLSL < 300 shaders that would otherwise run fine in GLES3 are downgraded to GLES2 and break.

GLES2 is not supported anymore and, after the recent updates to the engine, GLES3.0 is guaranteed in every platform, so having to deal with GLES2 and all its quirkiness just to support a lower default is not worth.

This PR move the default to GLSL300 when using a GLES backend.

If a shader is written with the old syntax, after this PR it needs to import GLSLCompat.glsllib, but this technically already the case in jme, since without GLSLCompat the shader will not work properly anyway.

[gemini-code-assist]

Code Review

This pull request updates the GLSL shader versioning logic in GLRenderer.java to support GLES3, upgrading versions between 150 and 300 to 300, and forcing #version 300 es for older shaders under GLES3. However, forcing GLSL 100 shaders to compile under #version 300 es will cause compilation failures due to deprecated keywords like attribute and varying which are unsupported in GLSL ES 3.00. Additionally, using String.valueOf(version) inside StringBuilder.append() is redundant and should be simplified to avoid unnecessary string allocations.