Fix composition of render-to-texture widgets on big endian

Converting from QImage::Format_ARGB32 to QRhiTexture::RGBA8 requires
texture swizzling also on big endian.  But unlike on little endian it
is not a red/blue swap that is needed, but a rather rotation of the
alpha channel from the first component to the last.

Add a new swizzling mode to the backingstorecompose fragment shader,
and rename the uniform to reflect that it no longer covers only
red/blue swapping.

Change-Id: I001d275abdc88faaadea16e396ebe032b34e83f0
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
(cherry picked from commit d2fcccd3f1c5e3729c280a6f0a3e12dc9835477b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/gui/painting/shaders/backingstorecompose.frag b/src/gui/painting/shaders/backingstorecompose.frag
index e0c419840e..3b08ade035 100644
--- a/src/gui/painting/shaders/backingstorecompose.frag
+++ b/src/gui/painting/shaders/backingstorecompose.frag
@@ -7,7 +7,7 @@ layout(std140, binding = 0) uniform buf {
     mat4 vertexTransform;
     mat3 textureTransform;
     float opacity;
-    int swapRedBlue;
+    int textureSwizzle;
 };
 
 layout(binding = 1) uniform sampler2D textureSampler;
@@ -16,8 +16,10 @@ void main()
 {
     vec4 tmpFragColor = texture(textureSampler, v_texcoord);
     tmpFragColor.a *= opacity;
-    if (swapRedBlue == 0)
+    if (textureSwizzle == 0)
         fragColor = tmpFragColor;
+    else if(textureSwizzle == 2)
+        fragColor.argb = tmpFragColor;
     else
-        fragColor = tmpFragColor.bgra;
+        fragColor.bgra = tmpFragColor;
 }