[linux] Ensure EGL uses OpenGL ES 2.0 client API

platform/linux/src/headless_backend_egl.cpp

@@ -58,11 +58,16 @@ void HeadlessBackend::createContext() {
     EGLDisplay display_ = display->attribute<EGLDisplay>();
     EGLConfig& config = display->attribute<EGLConfig&>();
 
-    if (!eglBindAPI(EGL_OPENGL_API)) {
-        throw std::runtime_error("Error setting the EGL rendering API.\n");
-    }
+    // EGL initializes the context client version to 1 by default. We want to
+    // use OpenGL ES 2.0 which has the ability to create shader and program
+    // objects and also to write vertex and fragment shaders in the OpenGL ES
+    // Shading Language.
+    const EGLint attribs[] = {
+        EGL_CONTEXT_CLIENT_VERSION, 2,
+        EGL_NONE
+    };
 
-    EGLContext glContext = eglCreateContext(display_, config, EGL_NO_CONTEXT, NULL);
+    EGLContext glContext = eglCreateContext(display_, config, EGL_NO_CONTEXT, attribs);
     if (glContext == EGL_NO_CONTEXT) {
         mbgl::Log::Error(mbgl::Event::OpenGL, "eglCreateContext() returned error 0x%04x",
                          eglGetError());

platform/linux/src/headless_display_egl.cpp

@@ -1,4 +1,5 @@
 #include <mbgl/platform/default/headless_display.hpp>
+#include <mbgl/platform/log.hpp>
 #include <mbgl/util/string.hpp>
 
 #include <EGL/egl.h>
@@ -25,6 +26,11 @@ HeadlessDisplay::Impl::Impl() {
         throw std::runtime_error("eglInitialize() failed.\n");
     }
 
+    if (!eglBindAPI(EGL_OPENGL_ES_API)) {
+        mbgl::Log::Error(mbgl::Event::OpenGL, "eglBindAPI(EGL_OPENGL_ES_API) returned error %d", eglGetError());
+        throw std::runtime_error("eglBindAPI() failed");
+    }
+
     // This shouldn't matter as we're rendering to a framebuffer.
     const EGLint attribs[] = { EGL_NONE };
     if (!eglChooseConfig(display, attribs, &config, 1, &numConfigs) || numConfigs != 1) {

test/util/offscreen_texture.test.cpp

@@ -74,17 +74,26 @@ TEST(OffscreenTexture, RenderToTexture) {
     MBGL_CHECK_ERROR(glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA));
 
     Shader paintShader(R"MBGL_SHADER(
+#ifdef GL_ES
+precision mediump float;
+#endif
 attribute vec2 a_pos;
 void main() {
     gl_Position = vec4(a_pos, 0, 1);
 }
 )MBGL_SHADER", R"MBGL_SHADER(
+#ifdef GL_ES
+precision mediump float;
+#endif
 void main() {
     gl_FragColor = vec4(0, 0.8, 0, 0.8);
 }
 )MBGL_SHADER");
 
         Shader compositeShader(R"MBGL_SHADER(
+#ifdef GL_ES
+precision mediump float;
+#endif
 attribute vec2 a_pos;
 varying vec2 v_texcoord;
 void main() {
@@ -92,6 +101,9 @@ void main() {
     v_texcoord = (a_pos + 1.0) / 2.0;
 }
 )MBGL_SHADER", R"MBGL_SHADER(
+#ifdef GL_ES
+precision mediump float;
+#endif
 uniform sampler2D u_texture;
 varying vec2 v_texcoord;
 void main() {