meson: revise png loader meson script.

We hate spagettie code, not even build script.
this revise it to keep the build script consistency.

meson.build

@@ -20,6 +20,10 @@ if get_option('loaders').contains('svg') == true
     config_h.set10('THORVG_SVG_LOADER_SUPPORT', true)
 endif
 
+if get_option('loaders').contains('png') == true
+    config_h.set10('THORVG_PNG_LOADER_SUPPORT', true)
+endif
+
 if get_option('vectors').contains('avx') == true
     config_h.set10('THORVG_AVX_VECTOR_SUPPORT', true)
 endif
@@ -33,10 +37,6 @@ if get_option('log') == true
     message('Enable Log')
 endif
 
-if get_option('png') == true
-    config_h.set10('THORVG_PNG_LOADER_SUPPORT', true)
-endif
-
 configure_file(
     output: 'config.h',
     configuration: config_h

meson_options.txt

@@ -6,9 +6,9 @@ option('engines',
 
 option('loaders',
    type: 'array',
-   choices: ['', 'svg'],
+   choices: ['', 'svg', 'png'],
    value: ['svg'],
-   description: 'Enable Vector File Loader in thorvg')
+   description: 'Enable File Loaders in thorvg')
 
 option('vectors',
    type: 'array',
@@ -33,11 +33,6 @@ option('examples',
     value: false,
     description: 'Enable building examples')
 
-option('png',
-   type: 'boolean',
-   value: false,
-   description: 'Enable libpng based png loading')
-
 option('test',
    type: 'boolean',
    value: false,

src/loaders/meson.build

@@ -5,8 +5,9 @@ if get_option('loaders').contains('svg') == true
     message('Enable SVG Loader')
 endif
 
-if get_option('png') == true
+if get_option('loaders').contains('png') == true
     subdir('png')
+    message('Enable PNG Loader')
 endif
 
 subdir('raw')

src/loaders/png/meson.build

@@ -3,7 +3,10 @@ source_file = [
    'tvgPngLoader.cpp',
 ]
 
+png_dep = meson.get_compiler('cpp').find_library('libpng')
+
 subloader_dep += [declare_dependency(
     include_directories : include_directories('.'),
+    dependencies : png_dep,
     sources : source_file
 )]

src/meson.build

@@ -19,12 +19,6 @@ subdir('bindings')
 thread_dep = meson.get_compiler('cpp').find_library('pthread')
 thorvg_lib_dep = [common_dep, loader_dep, binding_dep, thread_dep]
 
-if get_option('png') == true
-    message('Enable libpng based png loading')
-    png_dep = meson.get_compiler('cpp').find_library('libpng')
-    thorvg_lib_dep += png_dep
-endif
-
 thorvg_lib = library(
 	'thorvg',
 	include_directories    : headers,