[effects] Make sure that all effects reset the streaming buffer

Summary:
The streaming buffer is shared so we have to reset it before rendering
something. Otherwise, it's basically undefined behavior.

For example, if the streaming buffer had been used for rendering a
geometry with some color, then the Background Contrast effect may crash
in GLVertexBuffer::bindArrays, it all depends on whether there is any
bounded GLShader. To fix that, the buffer has to be reset.

BUG: 356141

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19034

effects/backgroundcontrast/contrast.cpp

@@ -483,6 +483,7 @@ void ContrastEffect::doContrast(EffectWindow *w, const QRegion& shape, const QRe
 
     // Upload geometry for the horizontal and vertical passes
     GLVertexBuffer *vbo = GLVertexBuffer::streamingBuffer();
+    vbo->reset();
     uploadGeometry(vbo, actualShape);
     vbo->bindArrays();
 

effects/blur/blur.cpp

@@ -647,6 +647,7 @@ void BlurEffect::doBlur(const QRegion& shape, const QRect& screen, const float o
 
     // Upload geometry for the down and upsample iterations
     GLVertexBuffer *vbo = GLVertexBuffer::streamingBuffer();
+    vbo->reset();
 
     uploadGeometry(vbo, expandedBlurRegion.translated(xTranslate, yTranslate), shape);
     vbo->bindArrays();