Use OpenGL ES 2.0 specific definitions for extension-only functionality.

If targetting explicitly OpenGL ES 2.0, only gl2.h is included (not
gl3.h).

src/AbstractShaderProgram.h

@@ -1058,7 +1058,11 @@ template<> struct Attribute<GLfloat> {
         Short = GL_SHORT,
         UnsignedInt = GL_UNSIGNED_INT,
         Int = GL_INT,
+        #ifndef MAGNUM_TARGET_GLES2
         HalfFloat = GL_HALF_FLOAT,
+        #else
+        HalfFloat = GL_HALF_FLOAT_OES,
+        #endif
         Float = GL_FLOAT
 
         #ifndef MAGNUM_TARGET_GLES
@@ -1104,7 +1108,11 @@ template<> struct Attribute<Math::Vector<4, GLfloat>> {
         Short = GL_SHORT,
         UnsignedInt = GL_UNSIGNED_INT,
         Int = GL_INT,
-        Half = GL_HALF_FLOAT,
+        #ifndef MAGNUM_TARGET_GLES2
+        HalfFloat = GL_HALF_FLOAT,
+        #else
+        HalfFloat = GL_HALF_FLOAT_OES,
+        #endif
         Float = GL_FLOAT
         #ifndef MAGNUM_TARGET_GLES
         ,

src/AbstractTexture.h

@@ -874,8 +874,12 @@ template<> struct MAGNUM_EXPORT AbstractTexture::DataHelper<2> {
 };
 template<> struct MAGNUM_EXPORT AbstractTexture::DataHelper<3> {
     enum class Target: GLenum {
+        #ifndef MAGNUM_TARGET_GLES2
         Texture3D = GL_TEXTURE_3D,
         Texture2DArray = GL_TEXTURE_2D_ARRAY
+        #else
+        Texture3D = GL_TEXTURE_3D_OES
+        #endif
     };
 
     inline constexpr static Target target() { return Target::Texture3D; }

src/Framebuffer.h

@@ -710,7 +710,11 @@ class MAGNUM_EXPORT Framebuffer {
              * @requires_gles30 %Extension @es_extension{APPLE,framebuffer_multisample}
              *      or @es_extension{ANGLE,framebuffer_blit}
              */
+            #ifndef MAGNUM_TARGET_GLES2
             Read = GL_READ_FRAMEBUFFER,
+            #else
+            Read = GL_READ_FRAMEBUFFER_APPLE,
+            #endif
 
             /**
              * For drawing only.
@@ -718,7 +722,11 @@ class MAGNUM_EXPORT Framebuffer {
              * @requires_gles30 %Extension @es_extension{APPLE,framebuffer_multisample}
              *      or @es_extension{ANGLE,framebuffer_blit}
              */
+            #ifndef MAGNUM_TARGET_GLES2
             Draw = GL_DRAW_FRAMEBUFFER,
+            #else
+            Draw = GL_DRAW_FRAMEBUFFER_APPLE,
+            #endif
 
             ReadDraw = GL_FRAMEBUFFER       /**< For both reading and drawing. */
         };

src/Magnum.h

@@ -25,7 +25,12 @@
 #include <GL/glew.h>
 #include <GL/glcorearb.h>
 #else
+#ifndef MAGNUM_TARGET_GLES2
 #include <GLES3/gl3.h>
+#else
+#include <GLES2/gl2.h>
+#include <GLES2/gl2ext.h>
+#endif
 #endif
 
 /**

src/Query.h

@@ -229,7 +229,11 @@ class MAGNUM_EXPORT SampleQuery: public AbstractQuery {
              * Whether any samples passed from fragment shader
              * @requires_gl33 Extension @extension{ARB,occlusion_query2}
              */
+            #ifndef MAGNUM_TARGET_GLES2
             AnySamplesPassed = GL_ANY_SAMPLES_PASSED,
+            #else
+            AnySamplesPassed = GL_ANY_SAMPLES_PASSED_EXT,
+            #endif
 
             /**
              * Whether any samples passed from fragment shader (conservative)
@@ -238,7 +242,11 @@ class MAGNUM_EXPORT SampleQuery: public AbstractQuery {
              * test at the expense of some false positives.
              * @requires_gl43 Extension @extension{ARB,ES3_compatibility}
              */
+            #ifndef MAGNUM_TARGET_GLES2
             AnySamplesPassedConservative = GL_ANY_SAMPLES_PASSED_CONSERVATIVE
+            #else
+            AnySamplesPassedConservative = GL_ANY_SAMPLES_PASSED_CONSERVATIVE_EXT
+            #endif
         };
 
         #ifndef MAGNUM_TARGET_GLES