scroll-view-fade: Put the shader in another file

This doesn't make any changes to the shader outside of adding
a header; it simply makes changes in the future easier to edit.

https://bugzilla.gnome.org/show_bug.cgi?id=689249

src/st/st-scroll-view-fade.glsl

@@ -0,0 +1,74 @@
+/*
+ * st-scroll-view-fade.glsl: Edge fade effect for StScrollView
+ *
+ * Copyright 2010 Intel Corporation.
+ * Copyright 2011 Adel Gadllah
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU Lesser General Public License,
+ * version 2.1, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+uniform sampler2D tex;
+uniform float height;
+uniform float width;
+uniform float offset_bottom;
+uniform float offset_top;
+uniform float offset_right;
+uniform float offset_left;
+
+/*
+ * Used to pass the fade area to the shader
+ *
+ * [0][0] = x1
+ * [0][1] = y1
+ * [1][0] = x2
+ * [1][1] = y2
+ *
+ */
+uniform mat2 fade_area;
+
+void main ()
+{
+    vec4 color = cogl_color_in * texture2D (tex, vec2 (cogl_tex_coord_in[0].xy));
+    float y = height * cogl_tex_coord_in[0].y;
+    float x = width * cogl_tex_coord_in[0].x;
+    float ratio = 1.0;
+    float fade_bottom_start = fade_area[1][1] - offset_bottom;
+    float fade_right_start = fade_area[1][0] - offset_right;
+    float ratio_top = y / offset_top;
+    float ratio_bottom = (fade_area[1][1] - y)/(fade_area[1][1] - fade_bottom_start);
+    float ratio_left = x / offset_left;
+    float ratio_right = (fade_area[1][0] - x)/(fade_area[1][0] - fade_right_start);
+    bool in_scroll_area = fade_area[0][0] <= x && fade_area[1][0] >= x;
+    bool fade_top = y < offset_top && in_scroll_area && (y >= fade_area[0][1]);
+    bool fade_bottom = y > fade_bottom_start && in_scroll_area && (y <= fade_area[1][1]);
+    bool fade_left = x < offset_left && in_scroll_area && (x >= fade_area[0][0]);
+    bool fade_right = x > fade_right_start && in_scroll_area && (x <= fade_area[1][0]);
+
+    if (fade_top) {
+        ratio *= ratio_top;
+    }
+
+    if (fade_bottom) {
+        ratio *= ratio_bottom;
+    }
+
+    if (fade_left) {
+        ratio *= ratio_left;
+    }
+
+    if (fade_right) {
+        ratio *= ratio_right;
+    }
+
+    cogl_color_out = color * ratio;
+}