gsk: Add a shader for external textures

This shader uses samplerExternalOES to sample an external texture
and blit it into a 'normal' texture. It only works in GLES, but
we won't use it outside of GLES.

gsk/gl/gskglprograms.defs

@@ -87,3 +87,13 @@ GSK_GL_DEFINE_PROGRAM (unblurred_outset_shadow,
                        GSK_GL_ADD_UNIFORM (1, UNBLURRED_OUTSET_SHADOW_SPREAD, u_spread)
                        GSK_GL_ADD_UNIFORM (2, UNBLURRED_OUTSET_SHADOW_OFFSET, u_offset)
                        GSK_GL_ADD_UNIFORM (3, UNBLURRED_OUTSET_SHADOW_OUTLINE_RECT, u_outline_rect))
+
+/* Texture conversion shaders.
+ *
+ * Note: If you add new formats here, they need to be added
+ * to the list of supported formats in gdk/gdkdmabuftexture.c.
+ */
+
+GSK_GL_DEFINE_PROGRAM_NO_CLIP (external,
+                               GSK_GL_SHADER_SINGLE (GSK_GL_SHADER_RESOURCE ("external.glsl")),
+                               GSK_GL_ADD_UNIFORM (1, EXTERNAL_SOURCE, u_external_source))

gsk/gl/resources/external.glsl

@@ -0,0 +1,28 @@
+// VERTEX_SHADER:
+// external.glsl
+
+void main() {
+  gl_Position = u_projection * u_modelview * vec4(aPosition, 0.0, 1.0);
+
+  vUv = vec2(aUv.x, aUv.y);
+}
+
+// FRAGMENT_SHADER:
+// external.glsl
+
+#if defined(GSK_GLES) || defined(GSK_GLES3)
+uniform samplerExternalOES u_external_source;
+#else
+/* Just to make this compile, we won't use it without GLES */
+uniform sampler2D u_external_source;
+#endif
+
+void main() {
+/* Open-code this here, since GskTexture() expects a sampler2D */
+#if defined(GSK_GLES) || defined(GSK_LEGACY)
+  vec4 color = texture2D(u_external_source, vUv);
+#else
+  vec4 color = texture(u_external_source, vUv);
+#endif
+  gskSetOutputColor(color);
+}

gsk/meson.build

@@ -20,6 +20,7 @@ gsk_private_gl_shaders = [
   'gl/resources/custom.glsl',
   'gl/resources/filled_border.glsl',
   'gl/resources/mask.glsl',
+  'gl/resources/external.glsl',
 ]
 
 gsk_public_sources = files([