infra: add Meson GL backend option for examples

Introduce meson_options.txt with a gl_backend selector and
use it to choose GL vs GLES deps/flags; update the GL
target call signature in MultiCanvas for the new API.

Author: wenjieshen

meson_options.txt

@@ -0,0 +1,5 @@
+option('gl_backend',
+  type: 'combo',
+  choices: ['gl', 'gles'],
+  value: 'gl',
+  description: 'Select GL backend for examples (gl, gles)')

src/MultiCanvas.cpp

@@ -22,7 +22,7 @@
 
 #include "Example.h"
 
-#ifdef THORVG_GL_RASTER_SUPPORT
+#if defined(TVGEXAMPLE_GL_SUPPORTED) || defined(TVGEXAMPLE_GLES_SUPPORTED)
     #include <SDL2/SDL_opengl.h>
 #endif
 
@@ -117,7 +117,7 @@ void runSw()
 /* GL Engine Specific Setup                                             */
 /************************************************************************/
 
-#ifdef THORVG_GL_RASTER_SUPPORT
+#if defined(TVGEXAMPLE_GL_SUPPORTED) || defined(TVGEXAMPLE_GLES_SUPPORTED)
 
 typedef void (*PFNGLTEXIMAGE2DPROC)(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type, const GLvoid* pixels);
 typedef void (*PFNGLTEXPARAMETERIPROC)(GLenum target, GLenum pname, GLint param);
@@ -173,7 +173,7 @@ struct GLFrameBuffer
 
 void runGl()
 {
-#ifdef THORVG_GL_RASTER_SUPPORT
+#if defined(TVGEXAMPLE_GL_SUPPORTED) || defined(TVGEXAMPLE_GLES_SUPPORTED)
 
 #ifdef THORVG_GL_TARGET_GLES
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES);
@@ -208,7 +208,7 @@ void runGl()
         auto canvas = unique_ptr<tvg::GlCanvas>(tvg::GlCanvas::gen());
 
         // Pass the framebuffer id to the GlCanvas
-        tvgexam::verify(canvas->target(context, glFbo.fbo, SIZE, SIZE, tvg::ColorSpace::ABGR8888S));
+        tvgexam::verify(canvas->target(nullptr, nullptr, context, glFbo.fbo, SIZE, SIZE, tvg::ColorSpace::ABGR8888S));
 
         content(canvas.get());
         if (tvgexam::verify(canvas->draw())) {

src/meson.build

@@ -4,16 +4,25 @@ examples_dep = [thorvg_dep, dependency('sdl2', required: true)]
 compiler_flags = ['-DSDL_MAIN_HANDLED']
 
 # GL or GLES
+gl_backend = get_option('gl_backend')
 gl_dep = dependency('gl', required: false)
 gles_dep = dependency('glesv2', required: false)
+egl_dep = dependency('egl', required: false)
 
-if gl_dep.found()
-    examples_dep += gl_dep
-elif gles_dep.found()
-    examples_dep += gles_dep
-    compiler_flags += ['-DTVGEXAMPLE_GLES_SUPPORTED']
-else
-    message('Neither OpenGL nor OpenGL ES was found. Skipping GL examples.')
+if gl_backend == 'gl'
+    if gl_dep.found()
+        examples_dep += gl_dep
+        compiler_flags += ['-DTVGEXAMPLE_GL_SUPPORTED']
+    else
+        message('Requested OpenGL but it was not found. Skipping GL examples.')
+    endif
+elif gl_backend == 'gles'
+    if gles_dep.found() and egl_dep.found()
+        examples_dep += [gles_dep, egl_dep]
+        compiler_flags += ['-DTVGEXAMPLE_GLES_SUPPORTED']
+    else
+        message('Requested OpenGL ES but it was not found. Skipping GL examples.')
+    endif
 endif
 
 # WebGPU