tests/cogl: Migrate blend test

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2555>

src/tests/cogl/conform/meson.build

@@ -1,6 +1,7 @@
 cogl_tests = [
   'test-atlas-migration',
   'test-blend-strings',
+  'test-blend',
 ]
 
 cogl_test_conformance_includes = [

src/tests/cogl/conform/test-blend.c

@@ -0,0 +1,67 @@
+#include <cogl/cogl.h>
+
+#include <string.h>
+
+#include "tests/cogl-test-utils.h"
+
+static void
+paint (void)
+{
+  CoglPipeline *pipeline = cogl_pipeline_new (test_ctx);
+  int width = cogl_framebuffer_get_width (test_fb);
+  int half_width = width / 2;
+  int height = cogl_framebuffer_get_height (test_fb);
+  CoglVertexP2 tri0_vertices[] = {
+        { 0, 0 },
+        { 0, height },
+        { half_width, height },
+  };
+  CoglVertexP2C4 tri1_vertices[] = {
+        { half_width, 0, 0x80, 0x80, 0x80, 0x80 },
+        { half_width, height, 0x80, 0x80, 0x80, 0x80 },
+        { width, height, 0x80, 0x80, 0x80, 0x80 },
+  };
+  CoglPrimitive *tri0;
+  CoglPrimitive *tri1;
+
+  cogl_framebuffer_clear4f (test_fb, COGL_BUFFER_BIT_COLOR, 0, 0, 0, 0);
+
+  tri0 = cogl_primitive_new_p2 (test_ctx, COGL_VERTICES_MODE_TRIANGLES,
+                                3, tri0_vertices);
+  tri1 = cogl_primitive_new_p2c4 (test_ctx, COGL_VERTICES_MODE_TRIANGLES,
+                                  3, tri1_vertices);
+
+  /* Check that cogl correctly handles the case where we draw
+   * different primitives same pipeline and switch from using the
+   * opaque color associated with the pipeline and using a colour
+   * attribute with an alpha component which implies blending is
+   * required.
+   *
+   * If Cogl gets this wrong then then in all likelihood the second
+   * primitive will be drawn with blending still disabled.
+   */
+
+  cogl_primitive_draw (tri0, test_fb, pipeline);
+  cogl_primitive_draw (tri1, test_fb, pipeline);
+
+  test_utils_check_pixel_and_alpha (test_fb,
+                                    half_width + 5,
+                                    height - 5,
+                                    0x80808080);
+}
+
+static void
+test_blend (void)
+{
+  cogl_framebuffer_orthographic (test_fb, 0, 0,
+                                 cogl_framebuffer_get_width (test_fb),
+                                 cogl_framebuffer_get_height (test_fb),
+                                 -1,
+                                 100);
+
+  paint ();
+}
+
+COGL_TEST_SUITE (
+  g_test_add_func ("/blend", test_blend);
+)