vcl: Fix DrawMask implementation in OpenGL backend

Change-Id: Idc0bedaba5a4cea351f131d402c2b1093ac1c53c

vcl/opengl/gdiimpl.cxx

@@ -406,7 +406,7 @@ bool OpenGLSalGraphicsImpl::CreateMaskProgram( void )
     glBindAttribLocation( mnMaskProgram, GL_ATTRIB_POS, "position" );
     glBindAttribLocation( mnMaskProgram, GL_ATTRIB_TEX, "tex_coord_in" );
     mnMaskUniform = glGetUniformLocation( mnMaskProgram, "sampler" );
-    mnMaskColorUniform = glGetUniformLocation( mnMaskProgram, "mask" );
+    mnMaskColorUniform = glGetUniformLocation( mnMaskProgram, "color" );
 
     CHECK_GL_ERROR();
     return true;
@@ -844,7 +844,7 @@ void OpenGLSalGraphicsImpl::DrawTextureWithMask( OpenGLTexture& rTexture, OpenGL
     CHECK_GL_ERROR();
 }
 
-void OpenGLSalGraphicsImpl::DrawMask( OpenGLTexture& rMask, SalColor nMaskColor, const SalTwoRect& /*pPosAry*/ )
+void OpenGLSalGraphicsImpl::DrawMask( OpenGLTexture& rMask, SalColor nMaskColor, const SalTwoRect& pPosAry )
 {
     if( mnMaskProgram == 0 )
     {
@@ -858,7 +858,10 @@ void OpenGLSalGraphicsImpl::DrawMask( OpenGLTexture& rMask, SalColor nMaskColor,
     glActiveTexture( GL_TEXTURE0 );
     rMask.Bind();
 
-    //DrawTextureRect( rMask, pPosAry );
+    glEnable( GL_BLEND );
+    glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );
+    DrawTextureRect( rMask, pPosAry );
+    glDisable( GL_BLEND );
 
     rMask.Unbind();
     glUseProgram( 0 );

vcl/opengl/maskFragmentShader.glsl

@@ -7,15 +7,15 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
-precision mediump float;
 varying vec2 tex_coord;
 uniform sampler2D sampler;
-uniform vec4 color;"
+uniform vec4 color;
 
 void main() {
-   vec4 texel0;
-   texel0 = texture2D(sampler, tex_coord);
-   gl_FragColor = color * texel0.a;
+    vec4 texel0;
+    texel0 = texture2D(sampler, tex_coord);
+    gl_FragColor = color;
+    gl_FragColor.a = 1.0 - texel0.r;
 }
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */