make scissor() work the same way on all backends

scissor() works like on metal now, that is, it is disabled when a
render pass starts.

The GL backend already assumed this in debug mode. We cannot really run
into issues at the moment because every time we get a new 
MaterialInstance we set the scissor -- we do this both for the
color pass and the post-process passes. With this change we will be
able to skip setting the scissor altogether in a lot of cases.

filament/backend/src/opengl/OpenGLDriver.cpp

@@ -2917,6 +2917,9 @@ void OpenGLDriver::beginRenderPass(Handle<HwRenderTarget> rth,
     GLuint const fbo = gl.bindFramebuffer(GL_FRAMEBUFFER, rt->gl.fbo);
     CHECK_GL_FRAMEBUFFER_STATUS(utils::slog.e, GL_FRAMEBUFFER)
 
+    // each render-pass starts with a disabled scissor
+    gl.disable(GL_SCISSOR_TEST);
+
     if (gl.ext.EXT_discard_framebuffer
             && !gl.bugs.disable_invalidate_framebuffer) {
         AttachmentArray attachments; // NOLINT
@@ -2929,7 +2932,6 @@ void OpenGLDriver::beginRenderPass(Handle<HwRenderTarget> rth,
         // It's important to clear the framebuffer before drawing, as it resets
         // the fb to a known state (resets fb compression and possibly other things).
         // So we use glClear instead of glInvalidateFramebuffer
-        gl.disable(GL_SCISSOR_TEST);
         clearWithRasterPipe(discardFlags & ~clearFlags, { 0.0f }, 0.0f, 0);
     }
 
@@ -2945,7 +2947,6 @@ void OpenGLDriver::beginRenderPass(Handle<HwRenderTarget> rth,
     }
 
     if (any(clearFlags)) {
-        gl.disable(GL_SCISSOR_TEST);
         clearWithRasterPipe(clearFlags,
                 params.clearColor, (GLfloat)params.clearDepth, (GLint)params.clearStencil);
     }
@@ -2964,7 +2965,6 @@ void OpenGLDriver::beginRenderPass(Handle<HwRenderTarget> rth,
 
 #ifndef NDEBUG
     // clear the discarded (but not the cleared ones) buffers in debug builds
-    gl.disable(GL_SCISSOR_TEST);
     clearWithRasterPipe(discardFlags & ~clearFlags,
             { 1, 0, 0, 1 }, 1.0, 0);
 #endif

filament/backend/src/vulkan/VulkanDriver.cpp

@@ -1229,6 +1229,11 @@ void VulkanDriver::beginRenderPass(Handle<HwRenderTarget> rth, const RenderPassP
     VulkanCommandBuffer& commands = mCommands.get();
     VkCommandBuffer const cmdbuffer = commands.buffer();
 
+    // Scissor is reset with each render pass
+    // This also takes care of VUID-vkCmdDrawIndexed-None-07832.
+    VkRect2D const scissor{ .offset = { 0, 0 }, .extent = extent };
+    vkCmdSetScissor(cmdbuffer, 0, 1, &scissor);
+
     UTILS_NOUNROLL
     for (uint8_t samplerGroupIdx = 0; samplerGroupIdx < Program::SAMPLER_BINDING_COUNT;
             samplerGroupIdx++) {
@@ -1901,11 +1906,6 @@ void VulkanDriver::bindPipeline(PipelineState const& pipelineState) {
     mPipelineCache.bindLayout(pipelineLayout);
     mPipelineCache.bindPipeline(commands);
 
-    // Since we don't statically define scissor as part of the pipeline, we need to call scissor at
-    // least once. Context: VUID-vkCmdDrawIndexed-None-07832.
-    auto const& extent = rt->getExtent();
-    scissor({0, 0, extent.width, extent.height});
-
     FVK_SYSTRACE_END();
 }
 
@@ -1976,6 +1976,9 @@ void VulkanDriver::scissor(Viewport scissorBox) {
     VulkanCommandBuffer& commands = mCommands.get();
     VkCommandBuffer cmdbuffer = commands.buffer();
 
+    // TODO: it's a common case that scissor() is called with (0, 0, maxint, maxint)
+    //       we should maybe have a fast path for this and avoid vkCmdSetScissor() if possible
+
     // Set scissoring.
     // clamp left-bottom to 0,0 and avoid overflows
     constexpr int32_t maxvali  = std::numeric_limits<int32_t>::max();