render/gles2: unify fragment shader

Instead of having 3 different fragment shaders for textures and 1
more for quads, unify them all and compile different variants via
a SOURCE constant.

In the future, we will have more variant dimensions: pre-multiplied
alpha on/off, color transformation, etc. It will become inpractical
to have one file per combination.

Weston has a similar approach:
https://gitlab.freedesktop.org/wayland/weston/-/blob/main/libweston/renderer-gl/fragment.glsl

render/gles2/shaders/common.frag

@@ -0,0 +1,45 @@
+/* enum wlr_gles2_shader_source */
+#define SOURCE_SINGLE_COLOR 1
+#define SOURCE_TEXTURE_RGBA 2
+#define SOURCE_TEXTURE_RGBX 3
+#define SOURCE_TEXTURE_EXTERNAL 4
+
+#if !defined(SOURCE)
+#error "Missing shader preamble"
+#endif
+
+#if SOURCE == SOURCE_TEXTURE_EXTERNAL
+#extension GL_OES_EGL_image_external : require
+#endif
+
+precision mediump float;
+
+varying vec2 v_texcoord;
+
+#if SOURCE == SOURCE_TEXTURE_EXTERNAL
+uniform samplerExternalOES tex;
+#elif SOURCE == SOURCE_TEXTURE_RGBA || SOURCE == SOURCE_TEXTURE_RGBX
+uniform sampler2D tex;
+#elif SOURCE == SOURCE_SINGLE_COLOR
+uniform vec4 color;
+#endif
+
+#if SOURCE != SOURCE_SINGLE_COLOR
+uniform float alpha;
+#else
+const float alpha = 1.0;
+#endif
+
+vec4 sample_texture() {
+#if SOURCE == SOURCE_TEXTURE_RGBA || SOURCE == SOURCE_TEXTURE_EXTERNAL
+	return texture2D(tex, v_texcoord);
+#elif SOURCE == SOURCE_TEXTURE_RGBX
+	return vec4(texture2D(tex, v_texcoord).rgb, 1.0);
+#elif SOURCE == SOURCE_SINGLE_COLOR
+	return color;
+#endif
+}
+
+void main() {
+	gl_FragColor = sample_texture() * alpha;
+}

render/gles2/shaders/meson.build

@@ -2,10 +2,7 @@ embed = find_program('./embed.sh', native: true)
 
 shaders = [
 	'common.vert',
-	'quad.frag',
-	'tex_rgba.frag',
-	'tex_rgbx.frag',
-	'tex_external.frag',
+	'common.frag',
 ]
 
 foreach name : shaders

render/gles2/shaders/quad.frag

@@ -1,8 +0,0 @@
-precision mediump float;
-varying vec4 v_color;
-varying vec2 v_texcoord;
-uniform vec4 color;
-
-void main() {
-	gl_FragColor = color;
-}

render/gles2/shaders/tex_external.frag

@@ -1,10 +0,0 @@
-#extension GL_OES_EGL_image_external : require
-
-precision mediump float;
-varying vec2 v_texcoord;
-uniform samplerExternalOES texture0;
-uniform float alpha;
-
-void main() {
-	gl_FragColor = texture2D(texture0, v_texcoord) * alpha;
-}

render/gles2/shaders/tex_rgba.frag

@@ -1,8 +0,0 @@
-precision mediump float;
-varying vec2 v_texcoord;
-uniform sampler2D tex;
-uniform float alpha;
-
-void main() {
-	gl_FragColor = texture2D(tex, v_texcoord) * alpha;
-}

render/gles2/shaders/tex_rgbx.frag

@@ -1,8 +0,0 @@
-precision mediump float;
-varying vec2 v_texcoord;
-uniform sampler2D tex;
-uniform float alpha;
-
-void main() {
-	gl_FragColor = vec4(texture2D(tex, v_texcoord).rgb, 1.0) * alpha;
-}