[Impeller] Require the GLES multisampled_render_to_texture2 extension for offscreen MSAA (#56997)

Offscreen MSAA needs the ability to create multisample depth and stencil attachments.

Author: jason-simmons

impeller/renderer/backend/gles/capabilities_gles.cc

@@ -22,6 +22,10 @@ static const constexpr char* kNvidiaTextureBorderClampExt =
 static const constexpr char* kMultisampledRenderToTextureExt =
     "GL_EXT_multisampled_render_to_texture";
 
+// https://registry.khronos.org/OpenGL/extensions/EXT/EXT_multisampled_render_to_texture2.txt
+static const constexpr char* kMultisampledRenderToTexture2Ext =
+    "GL_EXT_multisampled_render_to_texture2";
+
 CapabilitiesGLES::CapabilitiesGLES(const ProcTableGLES& gl) {
   {
     GLint value = 0;
@@ -123,10 +127,12 @@ CapabilitiesGLES::CapabilitiesGLES(const ProcTableGLES& gl) {
   if (desc->HasExtension(kMultisampledRenderToTextureExt)) {
     supports_implicit_msaa_ = true;
 
-    // We hard-code 4x MSAA, so let's make sure it's supported.
-    GLint value = 0;
-    gl.GetIntegerv(GL_MAX_SAMPLES_EXT, &value);
-    supports_offscreen_msaa_ = value >= 4;
+    if (desc->HasExtension(kMultisampledRenderToTexture2Ext)) {
+      // We hard-code 4x MSAA, so let's make sure it's supported.
+      GLint value = 0;
+      gl.GetIntegerv(GL_MAX_SAMPLES_EXT, &value);
+      supports_offscreen_msaa_ = value >= 4;
+    }
   }
   is_es_ = desc->IsES();
   is_angle_ = desc->IsANGLE();

impeller/renderer/backend/gles/test/capabilities_unittests.cc

@@ -63,5 +63,16 @@ TEST(CapabilitiesGLES, SupportsFramebufferFetch) {
   EXPECT_TRUE(capabilities->SupportsFramebufferFetch());
 }
 
+TEST(CapabilitiesGLES, SupportsMSAA) {
+  auto const extensions = std::vector<const unsigned char*>{
+      reinterpret_cast<const unsigned char*>(
+          "GL_EXT_multisampled_render_to_texture"),
+  };
+  auto mock_gles = MockGLES::Init(extensions);
+  auto capabilities = mock_gles->GetProcTable().GetCapabilities();
+  EXPECT_TRUE(capabilities->SupportsImplicitResolvingMSAA());
+  EXPECT_FALSE(capabilities->SupportsOffscreenMSAA());
+}
+
 }  // namespace testing
 }  // namespace impeller