infra: add Meson GL backend option for examples

[wenjieshen]

• Add gl_backend Meson option to choose gl or gles and gate deps/flags.
• Include EGL when targeting GLES and define THORVG_GL_RASTER_SUPPORT.
• Update GlCanvas::target call in MultiCanvas.cpp to new signature.

[Copilot]

Pull request overview

This PR adds a configurable GL backend option to the Meson build system, allowing users to choose between OpenGL and OpenGL ES for the examples. The change improves build flexibility by making the graphics backend selection explicit rather than attempting to auto-detect both.

Changes:

• Introduced a new gl_backend Meson option with choices of 'gl' or 'gles'
• Updated build logic to conditionally include appropriate dependencies (EGL for GLES) and compiler flags
• Replaced generic THORVG_GL_RASTER_SUPPORT macro with backend-specific TVGEXAMPLE_GL_SUPPORTED and TVGEXAMPLE_GLES_SUPPORTED macros
• Updated GlCanvas::target() call to match new API signature with additional parameters

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
meson_options.txt	Defines the new gl_backend option with 'gl' and 'gles' choices
src/meson.build	Implements conditional dependency resolution and compiler flag setting based on gl_backend choice
src/MultiCanvas.cpp	Updates preprocessor guards and GlCanvas::target() call to align with new build configuration

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[hermet]

@wenjieshen Could you please separate PR with MultiCanvas.cpp fix? I think choosing the OpenGL option in the examples doesn't seem like practical.